Calcium-based catalyst, preparation method thereof and method for preparing biodiesel

Abstract

The invention relates to the technical field of catalyst preparation and biomass energy, and discloses a calcium-based catalyst, a preparation method thereof and a method for preparing biodiesel. The calcium-based catalyst is a K-CaO solid solution, and K is dissolved into CaO crystal lattices; wherein the mass percent of K in the K-CaO solid solution is 2.5 to 5.5 percent. The calcium-based catalyst provided by the invention is a solid solution in which K is dissolved into CaO crystal lattices, and the K destroys the agglomeration of CaO particles, so that the specific surface area can be increased, and the biodiesel conversion rate is further improved; in addition, the alkaline site density of the calcium-based catalyst is greatly improved, and the cyclic stability is relatively good.

Description

Technical Field

[0001] This invention relates to the fields of catalyst preparation and biomass energy technology, specifically to a calcium-based catalyst, its preparation method, and a method for preparing biodiesel. Background Technology

[0002] With increasing global energy demand and intensifying environmental pressures, biodiesel, as a renewable and clean fuel, has attracted significant attention regarding its production technology. Currently, biodiesel is mainly produced through transesterification, where animal and vegetable oils react with methanol under the action of a catalyst to produce fatty acid methyl esters (biodiesel) and glycerol. The choice and performance of the catalyst directly determine the reaction efficiency, cost, and environmental impact. In existing technological fields, although homogeneous catalysts offer fast reaction rates and high conversion rates (>95%), they generate 3-5 tons of alkaline / acidic wastewater per ton of biodiesel produced, with treatment costs reaching 1500-2000 RMB / ton. Furthermore, the catalyst dissolves in the reaction system and cannot be recycled. Heterogeneous solid catalysts are recyclable and produce no wastewater, but they have low specific surface area (<20 m² / g), insufficient density of acidic or basic sites (<1.0 mmol / g), and poor cycle stability. Traditional CaO exhibits an activity decrease of >50% after three uses. Summary of the Invention

[0003] The purpose of this invention is to overcome the problems of existing homogeneous catalysts being easily soluble in the reaction system, unable to be recycled, and causing serious pollution, as well as existing heterogeneous catalysts having small specific surface area, low conversion rate, and poor cycle stability. This invention provides a calcium-based catalyst, its preparation method, and a method for preparing biodiesel. This calcium-based catalyst turns waste into treasure. The calcium-based catalyst prepared using eggshells not only has a high biodiesel conversion rate but also high cycle stability.

[0004] To achieve the above objectives, the present invention provides a calcium-based catalyst, which is a K-CaO solid solution, wherein K is dissolved in the CaO lattice; wherein the mass percentage of K in the K-CaO solid solution is 2.5-5.5%.

[0005] Preferably, the specific surface area of ​​the calcium-based catalyst is 35-50 m². 2 / g.

[0006] Preferably, the density of basic sites in the calcium-based catalyst is 1.8-2.5 mmol / g.

[0007] A second aspect of the present invention provides a method for preparing a calcium-based catalyst, the method comprising the following steps: (1) The eggshells are crushed, soaked in acid solution and calcined for the first time to obtain the eggshell pretreated product; (2) The eggshell pretreatment product and potassium salt solution were subjected to ultrasonic cavitation reaction, and then the reaction product was centrifuged to obtain a solid product; (3) The solid product is dried and then calcined.

[0008] Preferably, in step (1), the acid solution used for soaking is a 3-8% acetic acid solution, and the soaking time is 20-40 minutes.

[0009] Preferably, in step (1), the conditions for the first calcination include: a temperature of 850-900℃ and a time of 2-3h.

[0010] Preferably, in step (2), the solid-liquid ratio of the eggshell pretreatment product and the potassium salt solution is 1g:10-20mL.

[0011] Preferably, in step (2), the potassium salt in the potassium salt solution is at least one of potassium nitrate, potassium chloride, and potassium acetate.

[0012] Preferably, in step (2), the conditions for the ultrasonic cavitation reaction include: ultrasonic frequency of 20-60KHz, reaction temperature of 60-80℃, reaction time of 1-2h, and reaction pressure of atmospheric pressure.

[0013] Preferably, in step (3), the drying process includes: drying at 50-80℃ for 2-4 hours, and then drying at 100-120℃ for 1-2 hours.

[0014] Preferably, in step (3), the conditions for the second calcination include: a temperature of 450-550°C and a time of 1-3 hours.

[0015] A third aspect of the present invention provides a calcium-based catalyst prepared by the method described above.

[0016] The fourth aspect of this invention provides a method for preparing biodiesel, the method comprising: pre-filtering waste oil, then subjecting the filtered waste oil to transesterification with methanol in the presence of a catalyst, and then separating the reaction products to obtain biodiesel and glycerol; wherein the catalyst is the calcium-based catalyst described above.

[0017] Preferably, the ratio of waste oil to methanol is 1:0.25-0.35.

[0018] Preferably, the ratio of waste oil to catalyst is 1:0.03-0.05.

[0019] Preferably, the conditions for the transesterification reaction include: a temperature of 60-70°C and a time of 1.5-2.5 h.

[0020] Compared with the prior art, the present invention has at least the following beneficial effects: (1) The calcium-based catalyst provided by the present invention is a solid solution in which K is dissolved in the CaO lattice. Since K destroys the agglomeration of CaO particles, the specific surface area can be increased, thereby improving the biodiesel conversion rate (greater than 90%). In addition, the density of alkaline sites of the calcium-based catalyst is greatly improved, and the cycle stability is good.

[0021] (2) The method described in this invention uses household waste eggshells and nitrates as raw materials, and uses ultrasonic cavitation technology to generate micron-sized bubbles and break them instantly, forming local high temperature and high pressure, which promotes the insertion of K⁺ ions into the CaO lattice, destroys the aggregation of CaO particles, and has a large specific surface area. At the same time, the density of alkaline sites of the prepared calcium-based catalyst is greatly improved.

[0022] (3) The calcium-based catalyst provided by the present invention is used for the transesterification reaction of waste oil and methanol to obtain biodiesel and glycerol. The biodiesel has a high conversion rate and high cycle stability. Detailed Implementation

[0023] The following detailed description of specific embodiments of the present invention is provided in conjunction with the specific embodiments described herein. It should be understood that the specific embodiments described herein are for illustrative and explanatory purposes only and are not intended to limit the present invention.

[0024] The endpoints and any values ​​of the ranges disclosed herein are not limited to the precise ranges or values, and these ranges or values ​​should be understood to include values ​​close to these ranges or values. For numerical ranges, the endpoint values ​​of the various ranges, the endpoint values ​​of the various ranges and individual point values, and individual point values ​​can be combined with each other to obtain one or more new numerical ranges, which should be considered as specifically disclosed herein.

[0025] The calcium-based catalyst provided by this invention is a heterogeneous catalyst, which is a K-CaO solid solution containing three elements: K, Ca, and O, with K dissolved within the CaO crystal lattice. Because K is dissolved within the CaO crystal lattice, the calcium-based catalyst provided by this invention disrupts the aggregation of CaO particles, thereby increasing the specific surface area and improving the conversion rate of biodiesel from animal and vegetable oils via methanol transesterification. Furthermore, this calcium-based catalyst has a high density of alkaline sites, resulting in better cycle stability of the biodiesel produced.

[0026] Preferably, the mass percentage of K in the K-CaO solid solution can be 2.5-5.5 wt%. Controlling the mass percentage of K within this range ensures that K... + It effectively embeds itself into the CaO lattice to form a stable solid solution structure.

[0027] Furthermore, the specific surface area of ​​the calcium-based catalyst is 35-50 m².2 / g. Compared to conventional CaO catalysts (low specific surface area <20m² / g), the specific surface area of ​​the calcium-based catalyst is significantly increased.

[0028] Furthermore, the basic site density of the calcium-based catalyst is 1.8-2.5 mmol / g. Compared to conventional CaO catalysts (basic site density is approximately 25 m² / g), the basic site density of the calcium-based catalyst is significantly increased.

[0029] This invention provides a simple method for preparing calcium-based catalysts. Specifically, the method for preparing calcium-based catalysts includes the following steps: (1) The eggshells are crushed, soaked in acid solution and calcined for the first time to obtain the eggshell pretreated product; (2) The eggshell pretreatment product and potassium salt solution were subjected to ultrasonic cavitation reaction, and then the reaction product was centrifuged to obtain a solid product; (3) The solid product is dried and then calcined.

[0030] In the method for preparing calcium-based catalysts provided by this invention, ultrasonic cavitation technology is used to make full use of eggshells, a type of household waste, in order to prepare calcium-based catalysts for the transesterification reaction of waste oil to produce biodiesel. This not only effectively treats eggshells but also provides a good solution to the problem of waste oil pollution.

[0031] The purpose of step (1) is to pre-treat the eggshells. The specific operations of crushing, soaking in acid solution and first calcination of the eggshells in step (1) include: rinsing the eggshells with tap water, then crushing them in a crusher, soaking them in acid solution to remove impurities, and then transferring them to a muffle furnace for high-temperature calcination activation (first calcination). After calcination, the pre-treated eggshells obtained are mainly composed of calcium oxide.

[0032] In one embodiment, in step (1), the acid solution used for soaking can be a 3-8% acetic acid solution, and the soaking time can be 20-40 minutes.

[0033] In another embodiment, in step (1), the conditions for the first calcination include: a temperature of 850-900℃ and a time of 2-3 hours. In step (1), the eggshell pretreatment product obtained after calcination is mainly composed of calcium oxide.

[0034] In this invention, the purpose of step (2) is to process the structure of calcium oxide. Specifically, this invention mixes the eggshell pretreatment product with the potassium salt solution and uses ultrasonic cavitation technology to generate micron-sized bubbles that burst instantly, forming local high temperature (5000K) and high pressure (1000atm), which promotes the insertion of K⁺ ions into the CaO lattice, destroys the aggregation of CaO particles, and increases the specific surface area of ​​CaO.

[0035] In some embodiments, in step (2), the solid-liquid ratio of the eggshell pretreatment product to the potassium salt solution is 1g:10-20mL. Controlling the ratio of the eggshell pretreatment product to the potassium salt solution within this range ensures that the eggshell pretreatment product (CaO particles) is fully wetted and dispersed optimally.

[0036] Further, in step (2), the potassium salt in the potassium salt solution is at least one of potassium nitrate, potassium chloride, and potassium acetate, preferably potassium nitrate. Further, the concentration of the potassium salt solution can be 10-15 wt%.

[0037] In some embodiments, the conditions for the ultrasonic cavitation reaction in step (2) include: an ultrasonic frequency of 20-60 kHz, a reaction temperature of 60-80°C, a reaction time of 1-2 hours, and a reaction pressure of atmospheric pressure. The ultrasonic cavitation reaction is carried out in an ultrasonic reactor, and the frequency of the ultrasonic waves is adjusted by manually regulating the power of the ultrasonic transducer to provide a large amount of energy for the reaction. Simultaneously, the reaction temperature and pressure are controlled by observing a temperature detector and a pressure controller. Further, after the ultrasonic cavitation reaction is completed, the reaction products are separated by centrifugation to obtain the desired solid-phase product.

[0038] In this invention, the drying process in step (3) includes: first drying at 50-80℃ for 2-4 hours, and then drying at 100-120℃ for 1-2 hours. The purpose of dividing the drying into two stages is to avoid catalyst cracking caused by high-temperature rapid drying and to improve the structural stability of the calcium-based catalyst.

[0039] In some embodiments, in step (3), the conditions for the second calcination include: a temperature of 450-550°C and a time of 1-3 hours. After the second calcination, residual organic matter can be removed, promoting the formation of K-CaO solid solution crystals, thereby obtaining K-CaO solid solution.

[0040] The method described in this invention utilizes the high energy of sound waves in ultrasonic cavitation technology to convert CaCO3, the main component of eggshells, into a novel calcium-based catalyst. This novel calcium-based catalyst can significantly improve the conversion rate of waste oil into biodiesel; at the same time, the calcium-based catalyst can be recycled, and the biodiesel produced has good cycle stability.

[0041] Furthermore, this invention also provides a calcium-based catalyst prepared by the method described above. The novel catalyst provided by this invention can be used for the transesterification reaction of waste oil and methanol to obtain biodiesel and glycerol. This achieves comprehensive resource utilization, recovers waste eggshells and waste cooking oil, and produces approximately 4.615 million tons of biodiesel annually based on a biodiesel conversion rate of 92.3%, which can replace approximately 4.15 million tons of petrochemical diesel. Simultaneously, it significantly reduces the environmental harm of the preparation process, thus possessing enormous economic and environmental value.

[0042] The present invention also provides a method for preparing biodiesel, the method comprising: pre-filtering waste oil, then subjecting the filtered waste oil to transesterification with methanol in the presence of a catalyst, and then separating the reaction products to obtain biodiesel and glycerol; wherein the catalyst is the calcium-based catalyst described above.

[0043] In this invention, the waste oil can be common waste oil containing animal and vegetable oils, such as restaurant waste oil. The purpose of preliminary filtration of the waste oil is to remove food residue and moisture.

[0044] In some embodiments, the mass ratio of the waste oil to the methanol is 1:0.25-0.35. Controlling the amounts of waste oil and methanol within this range ensures complete reaction (conversion rate > 90%) while suppressing saponification side reactions (< 1%).

[0045] In some embodiments, the mass ratio of the waste oil to the catalyst can be 1:0.03-0.05. Limiting the amount of catalyst within this range enables efficient utilization of catalytic active sites (specific surface area utilization rate > 95%) and avoids increased separation energy consumption due to emulsification.

[0046] In this invention, the transesterification reaction can be carried out in a reaction vessel. In some embodiments, the conditions for the transesterification reaction include a temperature of 60-70°C and a time of 1.5-2.5 h. Limiting the temperature and time of the transesterification reaction to this range can balance reaction kinetics and thermal stability (rate constant of 0.52 h⁻¹ at 65°C), and prevent a decrease in conversion rate caused by methanol vaporization or reverse reaction.

[0047] In the process of preparing biodiesel, the separation of reaction products includes: condensing the reaction products in a condenser, allowing them to stand, and then separating them into layers. The upper layer is biodiesel, and the lower layer is glycerol. Furthermore, the lower layer also contains methanol and a catalyst; that is, the lower layer is a mixture containing glycerol, methanol, and a catalyst.

[0048] In a preferred embodiment, the method for preparing biodiesel further includes purifying glycerol, specifically including: rotary evaporating the lower layer product to recover methanol and washing and drying the methanol with water, then adding hydrochloric acid solution to the lower layer product after removing methanol until the pH value is 4, centrifuging to separate the catalyst, obtaining glycerol with a purity >95%, and then mixing the glycerol with beeswax to obtain an environmentally friendly candle.

[0049] This invention utilizes an ultrasound-assisted acid effect device and a transesterification reaction. In the transesterification reaction section, a novel calcium-based catalyst is generated to fully react methanol with triglycerides in waste oil. While producing biodiesel, the byproducts of the side reaction are controlled within a harmless range to the environment. The crude glycerol generated by the side reaction can be used to produce environmentally friendly candles, generating huge economic benefits while significantly reducing environmental pollution.

[0050] This invention achieves a triple breakthrough in environmental, economic, and social benefits through technological innovation and model innovation, and is a model practice of circular economy and green development.

[0051] The present invention will be described in detail below through examples, but the scope of protection of the present invention is not limited thereto. Unless otherwise specified, the reagents used in the present invention are all common commercially available products.

[0052] Examples - Example 9 illustrate the preparation process of calcium-based catalysts.

[0053] Example 1 The eggshells were initially rinsed with tap water, then crushed in a pulverizer, and then soaked in a 5% acetic acid solution to remove impurities. Next, they were transferred to a muffle furnace and calcined at 880°C for 2.5 hours to activate them. After calcination, the pretreated eggshell product was obtained. The eggshell pretreatment product was mixed with a 12wt% potassium nitrate solution, and then the mixture was fed into an ultrasonic reactor for ultrasonic cavitation reaction. The frequency of the ultrasonic waves was adjusted by manually regulating the power of the ultrasonic transducer to provide a large amount of energy for the reaction. At the same time, the reaction temperature and reaction pressure were controlled by observing the temperature detector and the pressure controller. The ultrasonic frequency was 20KHz, the reaction temperature was 70℃, the reaction time was 1.5 hours, and the reaction pressure was atmospheric pressure. After the reaction was completed, the reaction product was centrifuged in a centrifuge to obtain a solid product; The solid product was first dried at 60°C for 3 hours to remove most of the moisture, and then dried at 110°C for 1.5 hours to completely dry it. The dried product was then calcined at 500°C for 2 hours to obtain the calcium-based catalyst.

[0054] Example 2 The method was implemented according to Example 1, except that the ultrasonic frequency was 25 kHz.

[0055] Example 3 The method was implemented according to Example 1, except that the ultrasonic frequency was 30 kHz.

[0056] Example 4 The method was implemented according to Example 1, except that the ultrasonic frequency was 35 kHz.

[0057] Example 5 The method was implemented according to Example 1, except that the ultrasonic frequency was 40 kHz.

[0058] Example 6 The method was implemented according to Example 1, except that the ultrasonic frequency was 45 kHz.

[0059] Example 7 The method was implemented according to Example 1, except that the ultrasonic frequency was 50 kHz.

[0060] Example 8 The method was implemented according to Example 1, except that the ultrasonic frequency was 55 kHz.

[0061] Example 9 The method was implemented according to Example 1, except that the ultrasonic frequency was 60 kHz.

[0062] Test Example 1 According to the combined XRD and XRS tests, the products prepared in Examples 1-9 are solid solutions containing three elements: K, Ca, and O, and K is dissolved in the CaO lattice.

[0063] The K content in the products prepared in Examples 1-9 was tested using the XRF method, and the results are shown in Table 1.

[0064] Table 1 Test Example 2 The specific surface area of ​​the products prepared in Examples 1-9 was tested using the BET method, and the results are shown in Table 2.

[0065] Table 2 Test Example 3 The density of basic sites in the products prepared in Examples 1-9 was tested using the CO2-TPD method, and the results are shown in Table 3.

[0066] Table 3 Example 10 Waste cooking oil was initially filtered in a storage tank and then added to a reaction vessel. Methanol and the calcium-based catalyst prepared in Example 1 were added to the reaction vessel to carry out a transesterification reaction. The mass ratio of waste cooking oil to methanol was 1:0.3, the mass ratio of waste oil to catalyst was 1:0.04, the temperature of the transesterification reaction was 65°C, and the time was 2 hours. After the transesterification reaction is completed, the reaction product is condensed in a condenser. After standing, the reaction product separates into layers. The upper layer product is biodiesel, and the lower layer product is a mixture containing glycerol, methanol and catalyst. The lower layer product was subjected to rotary evaporation to recover methanol, and the methanol was washed with water and dried. Then, hydrochloric acid solution was added to the lower layer product after methanol removal until the pH value was 4. The calcium-based catalyst was separated by centrifugation to obtain glycerol with a purity >95%.

[0067] In this embodiment, the biodiesel conversion rate was 90.3%.

[0068] The separated calcium-based catalyst was recycled three times according to the aforementioned process, and the biodiesel conversion rates were 87.1%, 85.4%, and 83.2%, respectively.

[0069] The conversion rate is calculated as follows: Conversion rate (%) = Actual biodiesel mass ÷ (Federal oil mass × Theoretical yield coefficient) × 100.

[0070] Example 11 The method of Example 1 was followed, except that the calcium-based catalyst prepared in Example 1 was replaced with the calcium-based catalyst prepared in Example 2.

[0071] In this embodiment, the biodiesel conversion rate was 91.8%.

[0072] The separated calcium-based catalyst was recycled three times according to the aforementioned process, and the biodiesel conversion rates were 89.5%, 87.9%, and 86.3%, respectively.

[0073] Example 12 The method of Example 1 was followed, except that the calcium-based catalyst prepared in Example 1 was replaced with the calcium-based catalyst prepared in Example 3.

[0074] In this embodiment, the biodiesel conversion rate was 93.2%.

[0075] The separated calcium-based catalyst was recycled three times according to the aforementioned process, and the biodiesel conversion rates were 91.6%, 90.1%, and 88.7%, respectively.

[0076] Example 13 The method of Example 1 was followed, except that the calcium-based catalyst prepared in Example 1 was replaced with the calcium-based catalyst prepared in Example 4.

[0077] In this embodiment, the biodiesel conversion rate was 94.5%.

[0078] The separated calcium-based catalyst was recycled three times according to the aforementioned process, and the biodiesel conversion rates were 93.0%, 91.8%, and 90.5%, respectively.

[0079] Example 14 The method of Example 1 was followed, except that the calcium-based catalyst prepared in Example 1 was replaced with the calcium-based catalyst prepared in Example 5.

[0080] In this embodiment, the biodiesel conversion rate was 95.2%.

[0081] The separated calcium-based catalyst was recycled three times according to the aforementioned process, and the biodiesel conversion rates were 94.3%, 93.5%, and 92.7%, respectively.

[0082] Example 15 The method of Example 1 was followed, except that the calcium-based catalyst prepared in Example 1 was replaced with the calcium-based catalyst prepared in Example 6.

[0083] In this embodiment, the biodiesel conversion rate was 94.8%.

[0084] The separated calcium-based catalyst was recycled three times according to the aforementioned process, and the biodiesel conversion rates were 93.4%, 92.1%, and 90.9%, respectively.

[0085] Example 16 The method of Example 1 was followed, except that the calcium-based catalyst prepared in Example 1 was replaced with the calcium-based catalyst prepared in Example 7.

[0086] In this embodiment, the biodiesel conversion rate was 93.5%.

[0087] The separated calcium-based catalyst was recycled three times according to the aforementioned process, and the biodiesel conversion rates were 91.7%, 90.0%, and 88.4%, respectively.

[0088] Example 17 The method of Example 1 was followed, except that the calcium-based catalyst prepared in Example 1 was replaced with the calcium-based catalyst prepared in Example 8.

[0089] In this embodiment, the biodiesel conversion rate was 92.1%.

[0090] The separated calcium-based catalyst was recycled three times according to the aforementioned process, and the biodiesel conversion rates were 90.0%, 88.2%, and 86.5%, respectively.

[0091] Example 18 The method of Example 1 was followed, except that the calcium-based catalyst prepared in Example 1 was replaced with the calcium-based catalyst prepared in Example 9.

[0092] In this embodiment, the biodiesel conversion rate was 90.7%.

[0093] The separated calcium-based catalyst was recycled three times according to the aforementioned process, and the biodiesel conversion rates were 88.3%, 86.1%, and 84.0%, respectively.

[0094] Comparative Example 1 The method of Example 10 was followed, except that the calcium-based catalyst prepared in Example 1 was replaced with CaO.

[0095] In this comparative example, the biodiesel conversion rate was 82.3%.

[0096] The separated calcium-based catalyst was recycled three times according to the aforementioned process, and the biodiesel conversion rates were 70.5%, 65.0%, and 58.7%, respectively.

[0097] As can be seen from the examples and comparative examples, when the calcium-based catalyst prepared by the method described in this invention is used as a catalyst to prepare biodiesel, the conversion rate of biodiesel is greatly improved. Furthermore, the calcium-based catalyst can be recycled, resulting in better cycle stability of the prepared biodiesel.

[0098] It should be understood that any parts not described in detail in this specification belong to the prior art.

[0099] The preferred embodiments of the present invention have been described in detail above; however, the present invention is not limited thereto. Within the scope of the inventive concept, various simple modifications can be made to the technical solutions of the present invention, including combinations of various technical features in any other suitable manner. These simple modifications and combinations should also be considered as the content disclosed in the present invention and are all within the protection scope of the present invention.

Claims

1. A calcium-based catalyst, characterized in that, The calcium-based catalyst is a K-CaO solid solution, with K dissolved in the CaO lattice; wherein the mass percentage of K in the K-CaO solid solution is 2.5-5.5%.

2. The calcium-based catalyst according to claim 1, characterized in that, The specific surface area of ​​the calcium-based catalyst is 35-50 m². 2 / g; And / or, the density of basic sites in the calcium-based catalyst is 1.8-2.5 mmol / g.

3. A method for preparing a calcium-based catalyst, characterized in that, The method includes the following steps: (1) The eggshells are crushed, soaked in acid solution and calcined for the first time to obtain the eggshell pretreated product; (2) The eggshell pretreatment product and potassium salt solution were subjected to ultrasonic cavitation reaction, and then the reaction product was centrifuged to obtain a solid product; (3) Dry the solid product and then calcine it.

4. The method according to claim 3, characterized in that, In step (1), the acid solution used for soaking is a 3-8% acetic acid solution, and the soaking time is 20-40 minutes; And / or, in step (1), the conditions for the first calcination include: a temperature of 850-900℃ and a time of 2-3h.

5. The method according to claim 3 or 4, characterized in that, In step (2), the solid-liquid ratio of the eggshell pretreatment product and the potassium salt solution is 1g:10-20mL; And / or, in step (2), the potassium salt in the potassium salt solution is at least one of potassium nitrate, potassium chloride and potassium acetate; And / or, in step (2), the conditions for the ultrasonic cavitation reaction include: ultrasonic frequency of 20-60KHz, reaction temperature of 60-80℃, reaction time of 1-2h, and reaction pressure of atmospheric pressure.

6. The method according to claim 3, characterized in that, In step (3), the drying process includes: first drying at 50-80℃ for 2-4 hours, and then drying at 100-120℃ for 1-2 hours; And / or, in step (3), the conditions for the second calcination include: a temperature of 450-550°C and a time of 1-3 hours.

7. A calcium-based catalyst prepared by the method according to any one of claims 3-6.

8. A method for preparing biodiesel, characterized in that, The method includes: pre-filtering the waste oil, then subjecting the filtered waste oil to transesterification with methanol in the presence of a catalyst, and then separating the reaction products to obtain biodiesel and glycerol; The catalyst is a calcium-based catalyst according to any one of claims 1, 2 or 7.

9. The method according to claim 8, characterized in that, The mass ratio of the waste oil to the methanol used is 1:0.25-0.35; And / or, the mass ratio of the waste oil to the catalyst is 1:0.03-0.

05.

10. The method according to claim 8, characterized in that, The conditions for the transesterification reaction include: a temperature of 60-70℃ and a time of 1.5-2.5h.

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