Method for preparing high-purity biodiesel by one-step method based on synergistic catalysis of kitchen waste oil by magnetic bifunctional catalyst

Abstract

The invention particularly discloses a one-step method for preparing high-purity biodiesel based on synergistic catalysis of kitchen waste oil by a magnetic bifunctional catalyst. Comprising the following steps: (1) preparing a ferroferric oxide magnetic core, and coating the surface of the ferroferric oxide magnetic core with a porous silicon dioxide middle layer; through selective modification, a sulfonic acid group is loaded on the inner surface of the silicon dioxide shell as an acid center, and calcium magnesium oxide nanoparticles are loaded on the outer surface as an alkaline center, so that the catalyst is obtained; (2) putting the pretreated kitchen waste oil, short-chain alcohol and the catalyst into a reactor, and reacting for 2-4 hours at the temperature of 65-75 DEG C and the pressure of 0.3-0.8 MPa to obtain reaction liquid; and (3) sucking out the catalyst from the reaction liquid by using an external magnetic field, and carrying out standing, liquid separation and distillation on the reaction liquid to obtain the high-purity biodiesel. According to the invention, synergistic catalysis of acid and alkali active centers is realized on a single magnetic catalyst, and the problem of mutual neutralization of acid and alkali sites is solved.

Description

Technical Field

[0001] This application relates to the technical field of biodiesel preparation, and in particular to a one-step method for preparing high-purity biodiesel from kitchen waste oil based on the synergistic catalysis of magnetic bifunctional catalysts. Background Technology

[0002] Waste cooking oil typically refers to waste edible oils generated during food processing, cooking, and dining. It has a high acid value, high peroxide value, and contains harmful substances. If not properly handled, waste cooking oil can cause serious environmental and social problems.

[0003] Waste cooking oil can be transformed into a valuable resource through professional treatment. It is an ideal and inexpensive raw material for producing biodiesel. Through transesterification, the waste oil is converted into biodiesel. Biodiesel is a clean and renewable energy source, emitting far fewer pollutants than fossil diesel after combustion, effectively reducing greenhouse gas emissions.

[0004] However, the high free fatty acid content in kitchen waste oil is a "poison" for traditional alkaline catalysis methods. Existing technologies all have significant drawbacks: the homogeneous acid-base two-step process is complex, generates large amounts of wastewater, and corrodes equipment. Solid acid catalysis methods struggle to balance catalyst activity and stability, and separation and recovery from viscous products are extremely difficult (filter clogging, significant centrifugation losses), failing to meet the demands of continuous industrial production. Single-function catalysts can only catalyze either esterification or transesterification, unable to handle complex waste oil compositions, making a truly one-step process difficult.

[0005] Therefore, there is an urgent need to develop a new catalyst and supporting process that integrates high-efficiency catalysis, easy recovery, and adaptability to a wide range of raw materials. Summary of the Invention

[0006] To address the poor catalytic treatment effect of kitchen waste oil in existing technologies, this application provides a one-step method for preparing high-purity biodiesel from kitchen waste oil based on the synergistic catalysis of a magnetic bifunctional catalyst.

[0007] This application provides a one-step method for preparing high-purity biodiesel from kitchen waste oil based on the synergistic catalysis of a magnetic bifunctional catalyst, employing the following technical solution: A method for one-step preparation of high-purity biodiesel from kitchen waste oil based on the synergistic catalysis of a magnetic bifunctional catalyst includes the following steps: (1) Preparation of catalyst: First, a magnetic core of iron oxide was prepared and a porous silica intermediate layer was coated on its surface. Through selective modification, sulfonic acid groups were loaded on the inner surface of the silica shell as acidic centers and calcium magnesium oxide nanoparticles were loaded on the outer surface as basic centers to obtain the catalyst Fe3O4@SiO2-SO3H / CaO-MgO. (2) One-step reaction: The pretreated kitchen waste oil, short-chain alcohol and the above catalyst are placed in a reactor and reacted at a temperature of 65-75℃ and a pressure of 0.3-0.8MPa for 2-4 hours to obtain the reaction solution; (3) Product separation: After the reaction is completed, the catalyst is extracted from the reaction solution by an external magnetic field. The separation time is less than 3 minutes. The reaction solution is separated by standing and distillation to obtain high-purity biodiesel.

[0008] By employing the above technical solution, a magnetic bifunctional catalyst was prepared using a "sequential coating-regional loading" method. A magnetite core was prepared to provide strong magnetism, laying the foundation for rapid and low-cost separation and recovery of the catalyst. A porous silica interlayer was applied to prevent the internal magnetic core from being corroded or dissolved under reaction conditions, providing physical space for loading different active centers on the inner and outer surfaces, achieving "regional loading" and providing a larger loading area, which helps improve catalytic activity. The acidic centers (-SO3H) on the inner surface catalyze the esterification reaction of free fatty acids (FFAs) in waste cooking oil with methanol to produce fatty acid methyl esters (biodiesel). The alkaline centers (CaO-MgO) on the outer surface mainly catalyze the transesterification reaction of triglycerides in waste cooking oil with methanol to produce fatty acid methyl esters (biodiesel) and glycerol; the two work synergistically to achieve a one-step conversion.

[0009] In the same reactor, the two active centers of the catalyst work together: the basic center catalyzes the transesterification of triglycerides, while the acidic center simultaneously catalyzes the esterification of free fatty acids, thus avoiding the complex process and equipment switching of the traditional two-step method (first acid-catalyzed esterification, then base-catalyzed transesterification).

[0010] Using an external magnetic field, the solid catalyst is extracted from the reaction solution within 3 minutes. This method is much faster, more efficient, and less energy-intensive than traditional methods such as filtration and centrifugation. After the reaction solution is allowed to stand, it separates into two layers (the upper layer mainly consists of biodiesel and excess methanol, and the lower layer is the byproduct glycerol). The excess methanol is recovered by distillation to obtain high-purity biodiesel. In step (3), the catalyst can be recycled after simple washing and drying.

[0011] This application successfully solves the problem of saponification of high-acid-value kitchen waste oil in traditional alkaline catalysis. The acidic sites convert FFAs into biodiesel, preventing them from reacting with the alkaline catalyst to form soap, thereby improving yield and ensuring catalyst activity. Spatially isolating the acid and alkaline active centers effectively prevents their mutual neutralization and deactivation during the reaction, greatly improving catalyst stability and lifespan. This is key to achieving highly efficient synergistic catalysis. Magnetic field separation is a revolutionary catalyst recovery method. It has the advantages of high speed, simple operation, extremely low energy consumption, and minimal catalyst loss, aligning with the development direction of green chemistry and process intensification.

[0012] Preferred preparation of Fe3O4 magnetic core: FeCl3·6H2O and FeCl2·4H2O are dissolved in deoxygenated water, and ammonia is added dropwise under nitrogen protection with vigorous stirring at 60-62℃. After reacting for 30-33 min, magnetic separation is performed, and the core is washed 3-4 times each with water and ethanol, and then dried under vacuum.

[0013] By adopting the above technical solution, deoxygenated water is used to remove dissolved oxygen in the water to the maximum extent and avoid Fe... 2+ Oxidation. Nitrogen protection continuously prevents Fe from... 2+ Oxidation occurs. Ammonia water acts as a precipitant, providing OH- ions, which helps form Fe3O4 particles with better crystallinity and stronger magnetism. After the ammonia water is added, the reaction continues for 30 minutes, allowing the initially formed microcrystalline nuclei to grow and recombine, forming more stable particles with a more complete crystal structure. Utilizing the inherent magnetism of Fe3O4, the solid product is rapidly separated from the liquid by an external magnet, and the catalyst is easily recovered. A dry, loose, black Fe3O4 magnetic powder is obtained.

[0014] Preferred preparation of SiO2 coating (Fe3O4@SiO2): Fe3O4 magnetic cores are dispersed in a mixture of ethanol, water and ammonia, sonicated for 30-33 min, tetraethyl orthosilicate (TEOS) is slowly added dropwise, stirred at room temperature for 5.8-6 h, the product is magnetically separated, washed and dried.

[0015] By employing the above technical solution, ethanol serves as a co-solvent, aiding in the dissolution of TEOS; ammonia acts as a catalyst; and Fe3O4 nanoparticles, due to their magnetic properties, readily aggregate into large clusters. Ultrasonication effectively disperses these clusters, transforming them into individual, well-dispersed primary particles. Tetraethyl orthosilicate reacts with the surface of the Fe3O4 particles, achieving uniform coating and ensuring the formation of a complete and dense SiO2 shell. After SiO2 coating, the particles retain their superparamagnetism, facilitating easy separation using a magnet. Drying yields Fe3O4@SiO2 core-shell structured microspheres.

[0016] Preferred preparation of acidic site loading (Fe3O4@SiO2-SO3H): Fe3O4@SiO2 is dispersed in toluene, (3-mercaptopropyl)trimethoxysilane is added, refluxed at 105-110℃ for 12-13 h, cooled, magnetically separated, washed with toluene, and then the particles are dispersed in H2O2 solution for oxidation to convert -SH to -SO3H, magnetically separated, washed with water, and dried.

[0017] By employing the above technical solution, toluene, as an organic solvent, can effectively dissolve the silane coupling agent. The methoxy group of (3-mercaptopropyl)trimethoxysilane reacts with the silanol groups (-SiOH) on the SiO2 surface to form a strong Si-O-Si covalent bond, thereby "anchoring" the propyl chain with -SH to the SiO2 surface. Reflux ensures that the silanization reaction proceeds fully and completely, forming a dense and stable monolayer, which facilitates the diffusion of (3-mercaptopropyl)trimethoxysilane molecules into the pores and inner surface of the SiO2 shell, achieving "inner surface loading" of acidic centers. The product is recovered by magnetic separation and thoroughly washed with toluene to obtain Fe3O4@SiO2-SH. H2O2 selectively oxidizes the mercapto group (-SH) to a strongly acidic sulfonic acid group (-SO3H), and after drying, Fe3O4@SiO2-SO3H, i.e., magnetic microspheres loaded with acidic centers, is obtained.

[0018] Preferred preparation method: Fe3O4@SiO2-SO3H is impregnated in an ethanol solution containing Ca(NO3)2·4H2O and Mg(NO3)2·6H2O. After impregnation, the solution is dried by rotary evaporation and then calcined at 500-510℃ for 2.5-3h under N2 protection to obtain the final catalyst Fe3O4@SiO2-SO3H / CaO-MgO.

[0019] By employing the above technical solution, the solutions of calcium and magnesium nitrates are fully diffused and adsorbed onto the Fe3O4@SiO2-SO3H support. Since the pores and inner surface of the support are already occupied by the -SO3H groups and their long chains from the previous step, steric hindrance increases. Therefore, these metal salt precursors tend to be loaded onto the more open outer surface. Rotary evaporation ensures that the calcium and magnesium nitrates remain on the support surface as highly dispersed microcrystalline particles, rather than agglomerating. Calcination at 500℃ for 3 hours under N2 protection ensures complete decomposition of the nitrates into oxides while maintaining the stability of the SiO2 shell structure. The internal Fe3O4 magnetic cores are not oxidized under the protection of SiO2, forming composite basic centers. The successful introduction of basic active centers (CaO-MgO), together with the acidic centers (-SO3H) on the inner surface, enables the catalyst to simultaneously catalyze transesterification (base catalysis) and esterification (acid catalysis) reactions.

[0020] Preferably, the pretreated waste cooking oil is pretreated using the following method: (1) Solid-liquid separation and impurity removal: The kitchen waste oil is filtered or centrifuged to remove solid residues and suspended matter, resulting in preliminarily purified waste oil; (2) Dehydration and deacidification treatment: The waste oil obtained in step (1) is dehydrated at 60-80℃ and vacuum degree of 0.05-0.10MPa for 1-3 hours. Then, 0.5-2% of the mass of the waste oil is added and stirred to remove free fatty acids and trace amounts of water, and the treated waste oil is obtained. (3) Filtration: The waste oil treated in step (2) is filtered by pressure or centrifugation to remove the adsorbent and obtain pretreated kitchen waste oil.

[0021] By employing the above technical solution, solid-liquid separation and impurity removal are achieved, removing large particulate solid impurities such as food residue, bone fragments, and fibers. Dehydration is performed under conditions of 60-80℃ and 0.05-0.10MPa vacuum, avoiding oil oxidation caused by high temperatures. The adsorbent is deacidified to remove free fatty acids that result in high acid values, while further adsorbing trace amounts of moisture, pigments, and some colloidal impurities. Filtration removes the spent adsorbent after its adsorption function has been completed.

[0022] Preferably, the short-chain alcohol is one of methanol, ethanol, and n-butanol.

[0023] Preferably, an external magnetic field is applied, specifically as follows: A permanent magnet (neodymium iron boron magnet) is used as the magnetic field source. After the reaction is completed, the magnetic field source is placed close to the outer wall of the reactor, and the applied magnetic field strength is 0.3-1.2T for 2-10 minutes.

[0024] By adopting the above technical solution, neodymium iron boron is the material with the largest magnetic energy product and the strongest magnetic properties. It can provide a high-intensity magnetic field in a limited space, making it suitable for capturing nanoscale magnetic particles from reaction liquids.

[0025] Preferably, the reaction solution is allowed to stand and separate as follows: after removing the catalyst, the reaction solution is allowed to stand at 40-60℃ for 0.5-2 hours to allow it to naturally separate into an upper crude biodiesel phase and a lower excess short-chain alcohol-glycerol phase, and crude biodiesel is obtained by separation; the distillation conditions are: vacuum degree 0.01-0.05MPa, temperature 120-200℃, and the fraction is collected to obtain high-purity biodiesel.

[0026] In summary, this application has the following beneficial effects: 1. This application is the first to achieve “regional coexistence” and synergistic catalysis of acid and base active centers on a single magnetic catalyst, solving the problem of mutual neutralization of acid and base sites and showing universality for waste oil with complex composition.

[0027] 2. In this application, magnetic separation achieves near-zero loss recovery of the catalyst (recovery rate >99%), solving the global problem of recovering heterogeneous catalysts in biodiesel systems and significantly reducing production costs.

[0028] 3. This application achieves a one-step purification of high-acid-value feedstock through catalyst innovation. Compared with the traditional two-step method, equipment investment is reduced by more than 30%, energy consumption is reduced by more than 25%, and there is no emission of waste gas, wastewater, or solid waste. Excellent product performance: the yield of the obtained biodiesel is >96%, the purity meets the EN 14214 standard, and the purity of the glycerol byproduct is also higher. Detailed Implementation

[0029] The present application will be further described in detail below with reference to the embodiments.

[0030] The raw materials used in the examples and comparative examples are all commercially available. Example

[0031] Example 1 A method for one-step preparation of high-purity biodiesel from kitchen waste oil based on the synergistic catalysis of a magnetic bifunctional catalyst includes the following steps: (1) Catalyst preparation: A magnetic bifunctional catalyst was prepared by the "sequential coating-partition loading" method. First, a magnetic core of iron oxide was prepared and coated with a porous silica intermediate layer. Through selective modification, sulfonic acid groups were loaded on the inner surface of the silica shell as acidic centers and calcium magnesium oxide nanoparticles were loaded on the outer surface as basic centers to obtain the catalyst Fe3O4@SiO2-SO3H / CaO-MgO. (2) One-step reaction: 50g of pretreated kitchen waste oil with an acid value of 20mg KOH / g, methanol (molecular ratio of methanol to oil 18:1) and the above catalyst (6% of the mass of kitchen waste oil) were placed in a reactor and reacted at 70℃ and 0.5MPa for 3h to obtain the reaction solution. (3) Product separation: After the reaction is completed, the catalyst is extracted from the reaction solution by an external magnetic field. The separation time is 2 minutes. The reaction solution is separated by standing and distillation to obtain high-purity biodiesel.

[0032] Specifically, the preparation of Fe3O4 magnetic cores was carried out using a co-precipitation method: 5.2 g of FeCl3·6H2O and 2.0 g of FeCl2·4H2O were dissolved in 100 mL of deoxygenated water (deionized water was heated to boiling and maintained at a gentle boil for 15 min to remove most of the dissolved oxygen, followed by nitrogen purging and cooling to room temperature). Under nitrogen protection, 10 mL of ammonia water (28% by mass) was added dropwise while stirring vigorously at 60 °C (stirring speed 1000 rpm). After reacting for 30 min, magnetic separation was performed (after the reaction was completed, the reaction vessel was placed close to a 0.5T permanent magnet and allowed to stand for 5 minutes. At this time, the black Fe3O4 particles were attracted by the magnetic field and concentrated on the side of the container wall near the magnet. Then, the supernatant was carefully poured out or removed using a pipette, thus completing the solid-liquid separation). The mixture was washed three times each with water and ethanol, and then dried under vacuum at 60 °C to obtain the final product.

[0033] SiO2 coating (Fe3O4@SiO2): using Method: Disperse 0.5g of the above Fe3O4 magnetic cores in a mixture of 200mL ethanol, 50mL water and 5mL ammonia (mass concentration of 28%), sonicate for 30min, slowly add 2mL tetraethyl orthosilicate (TEOS), stir at room temperature for 6h, magnetically separate the product, wash and dry to obtain the final product.

[0034] Acidic site loading (Fe3O4@SiO2-SO3H): 0.5g Fe3O4@SiO2 was dispersed in 50mL toluene, 2mL (3-mercaptopropyl)trimethoxysilane was added, and the mixture was refluxed at 110℃ for 12h. After cooling, the mixture was magnetically separated, washed with toluene, and then the particles were dispersed in 50mL of 30% H2O2 solution and oxidized at 60℃ for 6h to convert -SH to -SO3H. The mixture was then magnetically separated, washed with water, and dried to obtain the final product.

[0035] Basic site loading (final product): 0.5 g Fe3O4@SiO2-SO3H was impregnated in 10 mL of an ethanol solution containing 0.1 g Ca(NO3)2·4H2O and 0.05 g Mg(NO3)2·6H2O for 4 h. After impregnation, it was dried by rotary evaporation (rotary evaporation at 60-80℃ water bath temperature and 40-60 rpm until dry (approximately 30-60 minutes)). Finally, it was calcined at 500℃ for 3 h under N2 protection to obtain the final catalyst Fe3O4@SiO2-SO3H / CaO-MgO.

[0036] The pretreatment method for waste cooking oil is as follows: (1) Solid-liquid separation and impurity removal: The kitchen waste oil is centrifuged to remove solid residues and suspended matter, and the waste oil is preliminarily purified; (2) Dehydration and deacidification treatment: The waste oil obtained in step (1) is dehydrated for 1 hour at 60°C and 0.10 MPa vacuum. Then, 2% of the waste oil mass of adsorbent (activated clay) is added and stirred to remove free fatty acids and trace amounts of water, and the treated waste oil is obtained. (3) Filtration: The waste oil treated in step (2) is centrifuged to remove the adsorbent and obtain pretreated kitchen waste oil.

[0037] An external magnetic field is applied, specifically as follows: A permanent magnet (neodymium iron boron magnet) is used as the magnetic field source. After the reaction is completed, the magnetic field source is placed close to the outer wall of the reactor, and the applied magnetic field strength is 1.2T for 2 minutes. Under the action of the magnetic field, the magnetic catalyst particles are agglomerated on the inner wall of the reactor. Then, the catalyst and the reaction liquid are separated into solid and liquid by pouring and filtration. The separated catalyst can be reused after being washed and dried with a low-carbon alcohol (ethanol).

[0038] The reaction solution after catalyst removal was allowed to stand at 60°C for 2 hours to allow it to naturally separate into an upper crude biodiesel phase and a lower excess short-chain alcohol-glycerol phase, thus obtaining crude biodiesel. The distillation conditions were: vacuum degree 0.05MPa, temperature 200℃. The distillate was collected to obtain high-purity biodiesel.

[0039] Example 2 A method for one-step preparation of high-purity biodiesel from kitchen waste oil based on the synergistic catalysis of a magnetic bifunctional catalyst, differing from Example 1 in that it includes the following steps: (1) Catalyst preparation: A magnetic bifunctional catalyst was prepared by the "sequential coating-partition loading" method. First, a magnetic core of iron oxide was prepared and coated with a porous silica intermediate layer. Through selective modification, sulfonic acid groups were loaded on the inner surface of the silica shell as acidic centers and calcium magnesium oxide nanoparticles were loaded on the outer surface as basic centers to obtain the catalyst Fe3O4@SiO2-SO3H / CaO-MgO. (2) One-step reaction: Pretreated kitchen waste oil, methanol and the above catalyst are placed in a reactor and reacted at 75°C and 0.3MPa for 2 hours to obtain a reaction solution; (3) Product separation: After the reaction is completed, the catalyst is extracted from the reaction solution by an external magnetic field. The separation time is 2 minutes. The reaction solution is separated by standing and distillation to obtain high-purity biodiesel.

[0040] Specifically, the preparation of the Fe3O4 magnetic core was carried out using a co-precipitation method: 5.7 g of FeCl3·6H2O and 1.5 g of FeCl2·4H2O were dissolved in 110 mL of deoxygenated water. Under nitrogen protection, 11 mL of ammonia water was added dropwise while vigorous stirring at 60 °C. After reacting for 30 min, magnetic separation was performed, and the core was washed three times each with water and ethanol, and then dried under vacuum at 60 °C to obtain the final product.

[0041] SiO2 coating (Fe3O4@SiO2): using Method: Disperse 0.3g of the above Fe3O4 magnetic cores in a mixture of 200mL ethanol, 50mL water and 6mL ammonia, sonicate for 30min, slowly add 1.8mL tetraethyl orthosilicate, stir at room temperature for 6h, magnetically separate the product, wash and dry to obtain the final product.

[0042] Acidic site loading (Fe3O4@SiO2-SO3H): 0.4 g Fe3O4@SiO2 was dispersed in 50 mL toluene, and 2.1 mL (3-mercaptopropyl)trimethoxysilane was added. The mixture was refluxed at 110 °C for 12 h. After cooling, the mixture was magnetically separated, washed with toluene, and then dispersed in 50 mL of 30% H2O2 solution. The mixture was oxidized at 60 °C for 6 h to convert -SH to -SO3H. The mixture was then magnetically separated, washed with water, and dried to obtain the final product.

[0043] Basic site loading (final product): 0.9 g Fe3O4@SiO2-SO3H was impregnated in 12 mL of ethanol solution containing 0.1 g Ca(NO3)2·4H2O and 0.06 g Mg(NO3)2·6H2O for 4 h. After impregnation, the solution was dried by rotary evaporation and finally calcined at 500 °C for 3 h under N2 protection to obtain the final catalyst Fe3O4@SiO2-SO3H / CaO-MgO.

[0044] The pretreatment method for waste cooking oil is as follows: (1) Solid-liquid separation and impurity removal: The kitchen waste oil is filtered to remove solid residues and suspended solids, resulting in preliminarily purified waste oil; (2) Dehydration and deacidification treatment: The waste oil obtained in step (1) is dehydrated for 2 hours at 70°C and 0.08 MPa vacuum. Then, 1.2% of the waste oil mass of adsorbent (activated clay) is added and stirred to remove free fatty acids and trace amounts of water, and the treated waste oil is obtained. (3) Filtration: The waste oil treated in step (2) is separated by pressure filtration to remove the adsorbent and obtain pretreated kitchen waste oil.

[0045] An external magnetic field is applied, specifically as follows: A permanent magnet (neodymium iron boron magnet) is used as the magnetic field source. After the reaction is completed, the magnetic field source is placed close to the outer wall of the reactor, and the applied magnetic field strength is 0.7T for 6 minutes. Under the action of the magnetic field, the magnetic catalyst particles are agglomerated on the inner wall of the reactor. Then, the catalyst and the reaction liquid are separated into solid and liquid by pouring and filtration. The separated catalyst can be reused after being washed and dried with a low-carbon alcohol (methanol).

[0046] The reaction solution after catalyst removal was allowed to stand at 50°C for 0.9 hours to allow it to naturally separate into an upper crude biodiesel phase and a lower excess short-chain alcohol-glycerol phase, thus obtaining crude biodiesel. The distillation conditions were: vacuum degree 0.03 MPa, temperature 150℃. The distillate was collected to obtain high-purity biodiesel.

[0047] Example 3 A method for one-step preparation of high-purity biodiesel from kitchen waste oil based on the synergistic catalysis of a magnetic bifunctional catalyst, differing from Example 1 in that it includes the following steps: (1) Catalyst preparation: A magnetic bifunctional catalyst was prepared by the "sequential coating-partition loading" method. First, a magnetic core of iron oxide was prepared and coated with a porous silica intermediate layer. Through selective modification, sulfonic acid groups were loaded on the inner surface of the silica shell as acidic centers and calcium magnesium oxide nanoparticles were loaded on the outer surface as basic centers to obtain the catalyst Fe3O4@SiO2-SO3H / CaO-MgO. (2) One-step reaction: Pretreated kitchen waste oil, ethanol and the above catalyst are placed in a reactor and reacted at 65°C and 0.8MPa for 4 hours to obtain a reaction solution; (3) Product separation: After the reaction is completed, the catalyst is extracted from the reaction solution by an external magnetic field. The separation time is 2 minutes. The reaction solution is separated by standing and distillation to obtain high-purity biodiesel.

[0048] Specifically, the preparation of the Fe3O4 magnetic core was carried out using a co-precipitation method: 4.8 g of FeCl3·6H2O and 2.5 g of FeCl2·4H2O were dissolved in 100 mL of deoxygenated water. Under nitrogen protection, 9 mL of ammonia water was added dropwise while vigorous stirring at 60 °C. After reacting for 30 min, magnetic separation was performed, and the core was washed three times each with water and ethanol, and then dried under vacuum at 60 °C to obtain the final product.

[0049] SiO2 coating (Fe3O4@SiO2): using Method: Disperse 0.7g of the above Fe3O4 magnetic cores in a mixture of 200mL ethanol, 50mL water and 4mL ammonia, sonicate for 30min, slowly add 2.2mL tetraethyl orthosilicate, stir at room temperature for 6h, magnetically separate the product, wash and dry to obtain the final product.

[0050] Acidic site loading (Fe3O4@SiO2-SO3H): 0.8 g Fe3O4@SiO2 was dispersed in 50 mL toluene, and 1.9 mL (3-mercaptopropyl)trimethoxysilane was added. The mixture was refluxed at 110 °C for 12 h. After cooling, the mixture was magnetically separated, washed with toluene, and then dispersed in 50 mL of 30% H2O2 solution. The mixture was oxidized at 60 °C for 6 h to convert -SH to -SO3H. The mixture was then magnetically separated, washed with water, and dried to obtain the final product.

[0051] Basic site loading (final product): 0.3 g Fe3O4@SiO2-SO3H was impregnated in 9 mL of an ethanol solution containing 0.1 g Ca(NO3)2·4H2O and 0.07 g Mg(NO3)2·6H2O for 4 h. After impregnation, the solution was dried by rotary evaporation and finally calcined at 500 °C for 3 h under N2 protection to obtain the final catalyst Fe3O4@SiO2-SO3H / CaO-MgO.

[0052] The pretreatment method for waste cooking oil is as follows: (1) Solid-liquid separation and impurity removal: The kitchen waste oil is filtered to remove solid residues and suspended solids, resulting in preliminarily purified waste oil; (2) Dehydration and deacidification treatment: The waste oil obtained in step (1) is dehydrated for 3 hours at 80°C and 0.05 MPa vacuum. Then, 0.5% of the waste oil mass of adsorbent (silica gel) is added and stirred to remove free fatty acids and trace amounts of water, and the treated waste oil is obtained. (3) Filtration: The waste oil treated in step (2) is separated by pressure filtration to remove the adsorbent and obtain pretreated kitchen waste oil.

[0053] An external magnetic field is applied, specifically as follows: A permanent magnet (neodymium iron boron magnet) is used as the magnetic field source. After the reaction is completed, the magnetic field source is placed close to the outer wall of the reactor, and the applied magnetic field strength is 0.3T for 10 minutes. Under the action of the magnetic field, the magnetic catalyst particles are agglomerated on the inner wall of the reactor. Then, the catalyst and the reaction liquid are separated into solid and liquid by pouring and filtration. The separated catalyst can be reused after being washed and dried with a low-carbon alcohol (methanol).

[0054] The reaction solution after catalyst removal was allowed to stand at 40°C for 0.5 hours to allow it to naturally separate into an upper crude biodiesel phase and a lower excess short-chain alcohol-glycerol phase, thus obtaining crude biodiesel. The distillation conditions were: vacuum degree 0.01MPa, temperature 120℃. The distillate was collected to obtain high-purity biodiesel.

[0055] Comparative Example Comparative Example 1 A method for preparing high-purity biodiesel from kitchen waste oil in one step based on the synergistic catalysis of a magnetic bifunctional catalyst differs from Example 1 in that it uses a magnetic catalyst (Fe3O4@SiO2-SO3H) with only acidic sites.

[0056] Comparative Example 2 A method for preparing high-purity biodiesel from kitchen waste oil in one step based on the synergistic catalysis of a magnetic bifunctional catalyst differs from Example 1 in that it uses a magnetic catalyst with only alkaline sites (Fe3O4@SiO2 / CaO-MgO).

[0057] Comparative Example 3 A method for preparing high-purity biodiesel from kitchen waste oil in a one-step process based on the synergistic catalysis of magnetic bifunctional catalysts differs from Example 1 in that magnetic solid acid catalyst (Fe3O4@SiO2-SO3H) of Comparative Example 1 and magnetic solid base catalyst (Fe3O4@SiO2 / CaO-MgO) of Comparative Example 2 are prepared respectively, ensuring that their preparation methods are completely consistent with the preparation methods of the corresponding parts in Example 1 of this invention.

[0058] The two catalyst powders were weighed precisely at a mass ratio of 1:1.

[0059] Place the two catalyst powders in a 50 mL sample vial and add an appropriate amount of inert solvent (n-hexane) until it just submerges the powder; the solvent is added to prevent dust from flying and to make the mixing more uniform.

[0060] The sample vials were placed in an ultrasonic cleaner and ultrasonically treated at room temperature for 30 minutes to ensure that the two catalysts were fully dispersed and in contact at the microscale.

[0061] The mixed slurry was dried in a vacuum drying oven at 60°C for 12 hours to completely remove the n-hexane solvent.

[0062] The dried block solid was gently ground in a mortar and passed through a 200-mesh sieve to obtain a uniform physical mixed catalyst powder, denoted as Fe3O4@SiO2-SO3H+Fe3O4@SiO2 / CaO-MgO(PM).

[0063] Comparative Example 4 A method for one-step preparation of high-purity biodiesel from kitchen waste oil based on the synergistic catalysis of a magnetic bifunctional catalyst differs from Example 1 in that the commercial solid acid catalyst Amberlyst-15 was purchased from Santa Cruz Biotechnology. 15(H).

[0064] Performance testing The methods for preparing high-purity biodiesel in Examples 1-3 and Comparative Examples 1-4 were subjected to performance testing. Acid value conversion rate: Refer to GB / T 7304-2014 "Determination of Acid Value of Petroleum Products and Lubricants (Potential Titration Method)"; Ester exchange rate: determined by infrared spectroscopy according to GB / T 23801-2009 "Determination of fatty acid methyl ester content in middle distillate oils"; Total biodiesel yield test method: determined by gravimetric method. Step: Accurately weigh the mass m of the pretreated feedstock oil. oil After reaction, separation, and distillation, the final refined biodiesel product mass m is accurately weighed. BD ; Yield = (m BD / m oil )×100%; Catalyst recovery (%): determined by gravimetric method; Procedure: Before the reaction, accurately weigh the mass m of the fresh catalyst used. cat After the fresh reaction, the catalyst is separated and recovered by an external magnetic field and washed with low-carbon alcohol.

[0065] The recovered catalyst was dried to constant weight in a vacuum drying oven at 60°C, and its mass m was accurately measured. cat The catalyst recovery rate (%) is calculated as follows: (m...) cat ,recovered / m cat ,fresh*100%; test results are shown in Table 1.

[0066] Table 1 Test data for the examples and comparative examples Table 2 Catalyst Cyclic Stability Test Loop count 1 2 3 4 5 Biodiesel yield (%) 96.8 96.0 95.1 93.5 91.0 As shown in Table 1, the high-purity biodiesel prepared in Examples 1-3 of this application exhibits good overall performance. Specifically, Example 1 achieved an acid conversion rate of 98.5%, a transesterification rate of 97.0%, and a total biodiesel yield of 96.8%, with catalyst recovery achieved via magnetic field and a recovery rate >99%. It is evident that the bifunctional catalyst of this application (Example 1) achieved a biodiesel yield significantly higher than that of the single-functional catalysts (Comparative Examples 1 and 2) and their physical mixture (Comparative Example 3), demonstrating the unexpected effect of synergistic catalysis at acidic and basic sites, rather than a simple addition. The magnetic separation method is significantly superior to traditional filtration methods in terms of recovery efficiency and ease of operation. Furthermore, the catalyst maintains high activity after 5 cycles, showcasing promising prospects for industrial application.

[0067] Comparative Example 2 showed that the alkalinity and saponification of FFA led to catalyst deactivation and a decrease in yield.

[0068] Table 2 shows the recycling performance of the catalyst prepared in Example 1. As can be seen from Table 2, the biodiesel yield after one cycle is 96.8%, and the biodiesel yield after five cycles is 91.0%, indicating that the catalyst prepared in this application has excellent recycling performance.

[0069] This specific embodiment is merely an explanation of this application and is not intended to limit it. After reading this specification, those skilled in the art can make modifications to this embodiment without contributing any inventive step, but such modifications are protected by patent law as long as they fall within the scope of the claims of this application.

Claims

1. A method for one-step preparation of high-purity biodiesel from kitchen waste oil based on the synergistic catalysis of a magnetic bifunctional catalyst, characterized in that, Includes the following steps: (1) Preparation of catalyst: First, a magnetic core of iron oxide is prepared and a porous silica intermediate layer is coated on its surface; through selective modification, sulfonic acid groups are loaded on the inner surface of the silica shell as acidic centers and calcium magnesium oxide nanoparticles are loaded on the outer surface as basic centers to obtain the catalyst Fe3O4@SiO2-SO3H / CaO-MgO. (2) One-step reaction: The pretreated kitchen waste oil, short-chain alcohol and the above catalyst are placed in a reactor and reacted at a temperature of 65-75℃ and a pressure of 0.3-0.8 MPa for 2-4 hours to obtain the reaction solution; (3) Product separation: After the reaction is completed, the catalyst is extracted from the reaction solution by an external magnetic field. The separation time is less than 3 minutes. The reaction solution is separated by standing and distillation to obtain high-purity biodiesel.

2. The method for one-step preparation of high-purity biodiesel from kitchen waste oil based on the synergistic catalysis of a magnetic bifunctional catalyst according to claim 1, characterized in that, Preparation of Fe3O4 magnetic core: FeCl3·6H2O and FeCl2·4H2O were dissolved in deoxygenated water. Under nitrogen protection, ammonia was added dropwise while stirring vigorously at 60-62℃. After reacting for 30-33 min, the core was magnetically separated, washed 3-4 times each with water and ethanol, and then dried under vacuum.

3. The method according to claim 1, wherein the method for preparing high purity biodiesel from waste cooking oil by one-step process based on magnetic bifunctional catalyst synergistic catalysis is characterized in that, Preparation of SiO2 coating: Fe3O4 magnetic cores were dispersed in a mixture of ethanol, water and ammonia, sonicated for 30-33 min, and tetraethyl orthosilicate was slowly added dropwise. The mixture was stirred at room temperature for 5.8-6 h, and the product was magnetically separated, washed and dried.

4. The method according to claim 1, wherein the method for preparing high-purity biodiesel from waste cooking oil by one-step process based on magnetic bifunctional catalyst synergistic catalysis is characterized in that, Preparation of acidic site loading: Fe3O4@SiO2 was dispersed in toluene, (3-mercaptopropyl)trimethoxysilane was added, and the mixture was refluxed at 105-110℃ for 12-13 h. After cooling, the mixture was magnetically separated, washed with toluene, and then the particles were dispersed in H2O2 solution for oxidation to convert -SH to -SO3H. The mixture was then magnetically separated, washed with water, and dried.

5. The method for one-step preparation of high-purity biodiesel from kitchen waste oil based on the synergistic catalysis of a magnetic bifunctional catalyst according to claim 1, characterized in that, Preparation of basic site loading: Fe3O4@SiO2-SO3H was impregnated in an ethanol solution containing Ca(NO3)2·4H2O and Mg(NO3)2·6H2O. After impregnation, the solution was dried by rotary evaporation and finally calcined at 500-510℃ for 2.5-3h under N2 protection to obtain the final catalyst Fe3O4@SiO2-SO3H / CaO-MgO.

6. The method according to claim 1, wherein the method for preparing high purity biodiesel from waste cooking oil by one-step process based on magnetic bifunctional catalyst synergistic catalysis is characterized in that, The pretreatment method for waste cooking oil is as follows: (1) Solid-liquid separation and impurity removal: The kitchen waste oil is filtered or centrifuged to remove solid residues and suspended matter, and the waste oil is preliminarily purified; (2) Dehydration and deacidification treatment: The waste oil obtained in step (1) is dehydrated at 60-80℃ and vacuum degree of 0.05-0.10 MPa for 1-3 hours. Then, 0.5-2% of the mass of the waste oil is added and stirred to remove free fatty acids and trace amounts of water, and the treated waste oil is obtained. (3) Filtration: The waste oil treated in step (2) is filtered by pressure or centrifugation to remove the adsorbent and obtain pretreated kitchen waste oil.

7. The method according to claim 1, wherein the method is characterized in that, Short-chain alcohols are one of methanol, ethanol, and n-butanol.

8. The method for one-step preparation of high-purity biodiesel from kitchen waste oil based on the synergistic catalysis of a magnetic bifunctional catalyst according to claim 1, characterized in that, An external magnetic field is applied, specifically as follows: A permanent magnet (neodymium iron boron magnet) is used as the magnetic field source. After the reaction is completed, the magnetic field source is placed close to the outer wall of the reactor, and the applied magnetic field strength is 0.3-1.2 T for 2-10 minutes.

9. The method for one-step preparation of high-purity biodiesel from kitchen waste oil based on the synergistic catalysis of a magnetic bifunctional catalyst according to claim 1, characterized in that, The reaction solution after catalyst removal is allowed to stand at 40-60℃ for 0.5-2 hours to allow it to naturally separate into an upper crude biodiesel phase and a lower excess short-chain alcohol-glycerol phase, thus obtaining crude biodiesel. The distillation conditions are: vacuum degree 0.01-0.05 MPa, temperature 120-200℃. The distillate is collected to obtain high-purity biodiesel.

Images