A bifunctional catalyst and its application in microwave-assisted preparation of biodiesel

Abstract

The application discloses a kind of bifunctional catalyst and its application in microwave-assisted preparation of biodiesel, belong to catalysis technical field.The application introduces cobalt metal in ZIF-8 skeleton, when MOFs derivative is prepared by calcination, the introduction of transition metal can construct magnetic center, adjust the magnetic loss ability of derivative material, so that the prepared catalyst has strong wave-absorbing performance under 2.45GHz industrial microwave field, can effectively absorb microwave energy and convert it into heat energy, thereby improving the energy utilization efficiency of reaction system, and finally improving the catalytic performance of catalyst, further improve the preparation efficiency of microwave-assisted biodiesel, and has good application value and prospect in catalysis technical field.

Description

Technical Field

[0001] This invention belongs to the field of catalysis technology, specifically relating to a bifunctional catalyst and its application in microwave-assisted biodiesel production. Background Technology

[0002] Biodiesel is typically produced through the esterification reaction of fatty acids and lower alcohols in the presence of an acidic catalyst. Traditional heating methods rely on heat conduction, which suffers from uneven heating, low energy efficiency, and long reaction times. Microwave heating, a volumetric heating method, couples the electromagnetic field with the polar molecules or conductive structures within the material at a fixed industrial frequency of 2.45 GHz, achieving rapid temperature rise. However, in microwave catalysis systems, reaction efficiency depends not only on the number of acidic sites but also heavily on the catalyst's ability to absorb and convert electromagnetic energy. Metal-organic framework (MOF) derivatives are functional materials obtained by high-temperature calcination of porous crystalline materials formed by metal ions and organic ligands through coordination bonds. They possess advantages such as high specific surface area, tunable pore size, and diverse chemical compositions, and have potential applications in catalysis. ZIF8 is a typical MOF material, with Zn as the central metal and 2-methylimidazole as the organic ligand. It exhibits excellent chemical and thermal stability, and its derivatives, after modification with ammonium sulfate, can be endowed with catalytically active sites. However, traditional ZIF-8 derivatives, with their single-metal Zn center, exhibit weak electromagnetic loss capability, and the magnetic and dielectric losses are difficult to match, resulting in insufficient absorption performance under 2.45 GHz industrial microwave fields. This limits their application in microwave-assisted biodiesel production catalytic systems. Therefore, how to construct a bifunctional material with excellent magnetic loss capability at 2.45 GHz and simultaneously possessing acid catalytic function to improve the efficiency of microwave-assisted biodiesel production is an urgent problem to be solved in this field. Summary of the Invention

[0003] This invention provides a bifunctional catalyst, which is a ZIF-8 (Zn / Co) catalyst, prepared by the following method:

[0004] 2-Methylimidazole was dissolved in methanol to obtain solution A; zinc nitrate hexahydrate and cobalt nitrate hexahydrate were dissolved in methanol to obtain solution B; solution A and solution B were mixed, stirred, allowed to stand for aging, and then filtered, washed and dried to obtain ZIF-8 (Zn / Co) precursor; under a nitrogen atmosphere, the ZIF-8 (Zn / Co) precursor was calcined for the first time, impregnated after calcination, and then calcined for the second time to obtain ZIF-8 (Zn / Co) catalyst.

[0005] In the above technical solution, the concentration of 2-methylimidazole in solution A is 0.05~0.3 mmol / mL, preferably 0.12 mmol / mL. In solution B, the concentration of zinc nitrate hexahydrate is 0.05~0.3 mmol / mL, preferably 0.1 mmol / mL; and the concentration of cobalt nitrate hexahydrate is 0.01~0.1 mmol / mL, preferably 0.04 mmol / mL.

[0006] In the above technical solution, the stirring conditions are: stirring at 30~45 ℃ for 5~20 h; the static aging conditions are: static aging at 30~45 ℃ for 5~20 h.

[0007] In the above technical solution, the conditions for the first calcination are: calcination at 800~1000 ℃ for 1~10 h; the conditions for the second calcination are: calcination at 300~500 ℃ for 1~10 h.

[0008] In the above technical solution, the immersion conditions are: immersion in 0.5~2 mol / L ammonium sulfate solution for 1~3 h.

[0009] This invention provides the application of the above-mentioned bifunctional catalyst in microwave-assisted biodiesel production.

[0010] This invention provides a method for preparing biodiesel based on microwave-assisted catalysis, comprising the following steps:

[0011] Fatty acids and alcohols are mixed, a bifunctional catalyst is added, and esterification is carried out under microwave irradiation to obtain biodiesel.

[0012] In the above biodiesel preparation method, the fatty acid is at least one of oleic acid, linoleic acid, palmitic acid, and linolenic acid; the alcohol is at least one of methanol, ethanol, and isopropanol.

[0013] In the above biodiesel preparation method, the molar ratio of fatty acids to alcohols is 1:(5~10); the amount of bifunctional catalyst used is 5~10% of the mass of oleic acid.

[0014] In the above biodiesel preparation method, the esterification reaction conditions are: microwave power 100~400 W, reaction temperature 30~100℃, and reaction time 30~200 min.

[0015] In the above biodiesel preparation method, the frequency of microwave irradiation is an industrial fixed frequency of 2.45 GHz.

[0016] The beneficial effects of this invention are as follows:

[0017] This invention introduces cobalt metal into the ZIF-8 framework. When preparing MOF derivatives through calcination, the introduction of the transition metal can construct magnetic centers and adjust the magnetic loss capability of the derivative material. This enables the prepared catalyst to have strong microwave absorption performance under a 2.45 GHz industrial microwave field, effectively absorbing microwave energy and converting it into heat energy, thereby improving the energy utilization efficiency of the reaction system and ultimately enhancing the catalytic performance of the catalyst. This further improves the efficiency of microwave-assisted biodiesel production and has good application value and prospects in the field of catalysis technology.

[0018] The catalyst described in this invention possesses both microwave absorption properties and acid catalytic activity, enabling a synergistic effect between the microwave heating process and the catalytic reaction process. This significantly improves the esterification reaction rate, thereby achieving a high biodiesel conversion rate in a shorter reaction time and realizing the efficient preparation of biodiesel. Attached Figure Description

[0019] Figure 1 Comparison of acid content of the bifunctional catalysts described in Examples 1-3.

[0020] Figure 2 A comparison of the conversion rates of microwave irradiation and water bath heating in the biodiesel esterification process.

[0021] Figure 3 The images show the Raman spectra of the bifunctional catalysts described in Examples 1-3.

[0022] Figure 4 The magnetization spectra are those of the bifunctional catalysts described in Examples 1-3. Detailed Implementation

[0023] Other materials used in this invention, unless otherwise stated, are commercially available. Other terms used in this invention, unless otherwise specified, generally have the meanings commonly understood by those skilled in the art. The invention is further described in detail below with reference to specific embodiments and data. The following embodiments are merely illustrative and not intended to limit the scope of the invention in any way.

[0024] Example 1

[0025] The ZIF-8 (Zn / Co) bifunctional catalyst was prepared by the following steps:

[0026] 12 mmol of 2-methylimidazole was dissolved in 100 mL of methanol to obtain solution A. 10 mmol of zinc nitrate hexahydrate and 4 mmol of cobalt nitrate hexahydrate were dissolved in 100 mL of methanol to obtain solution B. Solution A and solution B were mixed and stirred at 40 °C for 12 h, then allowed to stand at 40 °C for 12 h for aging. After filtration, washing, and drying, the ZIF-8 (Zn / Co) precursor was obtained. The precursor was calcined at 900 °C for 4 h under a nitrogen atmosphere, then impregnated with 1 mol / L ammonium sulfate solution for 1.5 h, followed by a second calcination at 400 °C for 4 h to obtain the ZIF-8 (Zn / Co) bifunctional catalyst.

[0027] Example 2

[0028] This embodiment prepared a ZIF-8 (Zn / Co) bifunctional catalyst. In this embodiment, the amount of cobalt nitrate hexahydrate was 3 mmol, and the other steps and conditions were the same as in Example 1.

[0029] Example 3

[0030] This embodiment prepared a ZIF-8 (Zn / Co) bifunctional catalyst. In this embodiment, the amount of cobalt nitrate hexahydrate was 2 mmol, and the other steps and conditions were the same as in Example 1.

[0031] I. Acidity determination

[0032] The determination of acidity is usually related to the number and distribution of acidic sites in the catalyst, which directly affects the activity of the catalytic reaction.

[0033] Weigh 0.2 g of catalyst and place it in 20 mL of NaCl solution (2 mol / L). Add 3-5 drops of phenolphthalein ethanol solution (10 g / L), shake thoroughly, and let stand for 2 h. Select NaOH solution (0.1 mol / L) as the titrant. Filter the solution after standing to obtain the filtrate and titrate it. The titration endpoint is reached when the indicator color no longer changes within 15 s, and record the volume of NaOH solution consumed at the titration endpoint. The formula for calculating the catalyst acidity (S) is as follows:

[0034]

[0035] In the formula, S represents the amount of catalyst acid, mmol / g; C N V represents the molar concentration of NaOH, in mol / L. N ρ is the titration volume of NaOH consumed, in mL; m is the mass of the catalyst, in g.

[0036] The measurement results are as follows Figure 1As shown: the catalyst in Example 1 had the lowest acid content, at 0.86 mmol / g, while the catalyst in Example 3 had the highest acid content, at 1.43 mmol / g.

[0037] II. Preparation of Biodiesel

[0038] 1. Microwave irradiation conditions

[0039] Oleic acid and methanol were mixed in a molar ratio of 1:8 to form a reaction system. The bifunctional catalyst prepared in the above example (catalyst amount was 8 wt.% of the mass of oleic acid) was added, and the esterification reaction was carried out under microwave irradiation conditions (frequency band) of 2.45 GHz, with a microwave power of 200 W, a reaction temperature of 60 ℃, and a reaction time of 120 min.

[0040] The biodiesel conversion rate was measured after the reaction was completed. The formula for calculating the esterification reaction conversion rate is as follows:

[0041]

[0042] Where C is the conversion rate (%), X0 is the acid value of the system before the reaction (mg KOH / g), and X1 is the acid value of the system after the reaction (mg KOH / g).

[0043] Calculations showed that the biodiesel conversion rate using the catalyst of Example 1 was 90.9%, the biodiesel conversion rate using the catalyst of Example 2 was 83%, and the biodiesel conversion rate using the catalyst of Example 3 was 78%.

[0044] 2. Water bath heating conditions

[0045] Oleic acid and methanol were mixed in a molar ratio of 1:8 to form a reaction system. The bifunctional catalyst prepared in the above example (catalyst amount was 8 wt.% of the mass of oleic acid) was added, and the esterification reaction was carried out under water bath heating conditions. The water bath reaction temperature was 60 °C and the reaction time was 120 min.

[0046] After the reaction was completed, the biodiesel conversion rate was measured. The biodiesel conversion rate using the catalyst of Example 1 was 63.9%, the biodiesel conversion rate using the catalyst of Example 2 was 67.2%, and the biodiesel conversion rate using the catalyst of Example 3 was 71.7%.

[0047] Figure 2 The conversion rates of microwave irradiation and water bath heating in the biodiesel esterification process are compared.

[0048] Under microwave irradiation conditions, Example 1 showed the highest biodiesel conversion rate of 90.9%; the conversion rates of Examples 2 and 3 were lower, at 83% and 78%, respectively.

[0049] Under water bath heating conditions, Example 1 showed the lowest conversion rate at 63.9%; while Examples 2 and 3 showed rates of 67.2% and 71.7% respectively, both significantly lower than those under microwave heating conditions.

[0050] The above results indicate that the catalytic reaction rate under water bath heating is limited by the heat transfer efficiency, making it difficult to achieve the same effect as microwave heating. Microwave-assisted heating significantly improves the reaction efficiency of the catalyst, especially the catalyst in Example 1. This suggests that under microwave heating conditions, the biodiesel conversion rate mainly depends on the catalyst's microwave absorption capacity rather than its acid content.

[0051] In summary, under microwave conditions, the reaction conversion rate is positively correlated with the catalyst magnetic loss parameter, while under conventional heating conditions it is mainly controlled by the amount of acid, indicating that the main controlling factor of the reaction changes in the microwave field.

[0052] III. Raman Spectroscopy

[0053] The Raman spectra of the bifunctional catalysts described in Examples 1-3 above were measured, as follows: Figure 3 As shown.

[0054] The positions of the D and G peaks are marked in the figure. They represent the characteristic signals of defects (D peak) and graphitized structures (G peak) in the carbon material of the catalyst, respectively. The ratio of the D peak to the G peak is usually used to determine the degree of graphitization of the catalyst. The ID / IG values ​​of Examples 1, 2, and 3 are 1.041, 1.036, and 1.039, respectively. The similar ratios indicate that the degree of graphitization in Examples 1-3 is similar, suggesting that adjusting the Zn / Co ratio does not significantly change the catalyst structure and will not significantly affect the catalyst's absorption capacity.

[0055] IV. Magnetization

[0056] The magnetic properties of the catalyst, including saturation magnetization (Ms) and coercivity (Hc), were measured using a SQUID-VSM vibrating sample magnetometer. The applied magnetic field range was -15000 Oe to 15000 Oe.

[0057] Test results are as follows Figure 4 As shown:

[0058] Figure 4The comparison of magnetization intensity of the bifunctional catalysts described in Examples 1-3 under different magnetic fields (unit: Oe) is shown. As shown in the figure, the saturation magnetization intensity of Examples 1-3 is 12.4 emu / g, 10.6 emu / g, and 7.6 emu / g, respectively. It can be seen that Example 1 has a stronger magnetic response, which enables it to absorb energy more effectively during microwave heating. In contrast, Examples 2 and 3 have weaker magnetic responses, which will affect the catalyst's microwave absorption capacity and catalytic efficiency.

[0059] V. Electromagnetic Characteristic Parameters

[0060] In microwave heating, electromagnetic properties are crucial to the performance of catalysts. By analyzing these properties, we can explain the catalyst's microwave absorption capacity, energy conversion efficiency, and other characteristics.

[0061] The electromagnetic parameters of the catalyst, including the complex permittivity, were tested using a P5000A vector network analyzer via the coaxial line method. , and complex permeability , .

[0062] Before testing, the catalyst and paraffin were thoroughly mixed at 100 °C at a mass fraction of 30 wt.%. The mixture was then pressed into a ring-shaped sample with an outer diameter of 7 mm, an inner diameter of 3.04 mm, and a thickness of 6 mm. After cooling and solidification, the sample was mounted in a coaxial test fixture to form a closed test loop. The complex permittivity and complex permeability of the sample at 2.45 GHz were calculated using Network Analyzer software, and the dielectric loss tangent was further obtained. and magnetic loss tangent .

[0063] The measurement results are shown in Table 1:

[0064] Table 1 Electromagnetic characteristic parameter data table

[0065]

[0066] As shown in Table 1, Example 1 exhibits the strongest microwave absorption capability and has a high magnetic loss tangent. Impedance matching (Z) enables efficient energy conversion under microwave heating conditions, improving catalytic reaction efficiency. However, the lower magnetic permeability of Examples 2 and 3 results in slightly lower microwave absorption and energy conversion efficiency.

[0067] The above description is merely a preferred embodiment of the present invention and is not intended to limit the invention in any other way. Any person skilled in the art may make changes or modifications to the above-disclosed technical content to create equivalent embodiments. However, any simple modifications, equivalent changes, and modifications made to the above embodiments based on the technical essence of the present invention without departing from the scope of the present invention shall still fall within the protection scope of the present invention.

Claims

1. A method for preparing biodiesel based on microwave-assisted catalysis, characterized in that, The steps are as follows: Fatty acids and alcohols are mixed, a bifunctional catalyst is added, and esterification is carried out under microwave irradiation to obtain biodiesel. The bifunctional catalyst is prepared by the following method: 2-Methylimidazole was dissolved in methanol to obtain solution A; zinc nitrate hexahydrate and cobalt nitrate hexahydrate were dissolved in methanol to obtain solution B; solution A and solution B were mixed, stirred, allowed to stand for aging, and then filtered, washed and dried to obtain ZIF-8 (Zn / Co) precursor; under a nitrogen atmosphere, the ZIF-8 (Zn / Co) precursor was calcined for the first time, impregnated after calcination, and then calcined for the second time to obtain a bifunctional catalyst; In solution A, the concentration of 2-methylimidazole is 0.05~0.3 mmol / mL; in solution B, the concentration of zinc nitrate hexahydrate is 0.05~0.3 mmol / mL, and the concentration of cobalt nitrate hexahydrate is 0.01~0.1 mmol / mL. The conditions for the first calcination are: calcination at 800~1000 ℃ for 1~10 h; the conditions for the second calcination are: calcination at 300~500 ℃ for 1~10 h. The immersion conditions are as follows: immersion in 0.5~2 mol / L ammonium sulfate solution for 1~3 h.

2. The method for preparing biodiesel based on microwave-assisted catalysis according to claim 1, characterized in that, The stirring conditions are: stirring at 30~45 ℃ for 5~20 h; the static aging conditions are: static aging at 30~45 ℃ for 5~20 h.

3. The method for preparing biodiesel based on microwave-assisted catalysis according to claim 1, characterized in that, The fatty acid is at least one of oleic acid, linoleic acid, palmitic acid, and linolenic acid; the alcohol is at least one of methanol, ethanol, and isopropanol.

4. The method for preparing biodiesel based on microwave-assisted catalysis according to claim 1, characterized in that, The molar ratio of the fatty acid to the alcohol is 1:(5~10); the amount of the bifunctional catalyst is 5~10% of the mass of oleic acid.

5. The method for preparing biodiesel based on microwave-assisted catalysis according to claim 1, characterized in that, The conditions for the esterification reaction are: microwave power 100~400 W, reaction temperature 30~100℃, and reaction time 30~200 min.

6. The method for preparing biodiesel based on microwave-assisted catalysis according to claim 1, characterized in that, The frequency of the microwave irradiation is a fixed industrial frequency of 2.45 GHz.

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