Substituted heteropoly acid catalyst, its preparation method and application

Abstract

This invention discloses a substituted heteropolyacid catalyst, its preparation method, and its application. This invention utilizes H3PW... 12 O 40 The potassium salt of the vacancy-deficient heteropolyacid K7PW was obtained by mixing KCl with ultrapure water, adjusting the pH to 5.5, filtering to remove insoluble matter, and drying. 11 O 39 ; K7PW 11 O 39 Dissolved in ultrapure water, 0.035 g / mL cobalt chloride solution was added dropwise at 80°C with stirring until the reaction was complete. The reaction product was cooled and filtered to remove insoluble matter. The resulting filtrate was added to a mixed solution of methanol and ethanol, and stirring was continued until precipitation was complete. The resulting filtrate was then precipitated, washed, dried, ground, and collected to obtain the substituted heteropolyacid potassium salt K5PW. 11 CoO 39 K5PW 11 CoO 39 Potassium ions were replaced with hydrogen ions via ion exchange, and the resulting product was collected after drying and grinding to obtain the substituted heteropolyacid catalyst H5PW. 11 CoO 39 This invention, H5PW 11 CoO 39 It can efficiently promote furfural transfer and synthesize isopropyl levulinate under mild conditions without the need to separate intermediate products in the reaction system, making it more environmentally friendly, lower cost and more efficient.

Description

Technical Field

[0001] This invention relates to the field of chemical synthesis technology, and in particular to a substituted heteropolyacid catalyst, its preparation method, and its application. Background Technology

[0002] With the depletion of fossil fuels and growing societal concern about global warming, many researchers are constantly searching for environmentally friendly and sustainable energy sources. Biomass, due to its wide availability and renewability, has attracted considerable attention. Biomass can be converted into fuels and high-value-added fine chemicals, effectively reducing dependence on fossil fuels. Furfural is an important biomass platform molecule, an organic compound containing a furan ring derived from pentose sugars. It can be produced commercially on a large scale from straw and other materials, and through various chemical reactions, it can be transformed into high-value-added chemicals, playing a significant role in the development and utilization of biomass resources. Furfural can be converted into high-value-added chemicals such as furfuryl alcohol and levulinic acid esters through oxidation, hydrogenation, and condensation reactions, achieving efficient utilization of biomass resources. Among these, levulinic acid esters can be used as additives in gasoline and diesel fuels, and can also be further converted through chemical reactions into chemical substances with functional groups such as aldehydes, ketones, and carboxylic acids, used in the production of important industrial chemicals such as plasticizers and biodegradable plastics, possessing enormous commercial and industrial value.

[0003] Traditional conversion methods utilize exogenous hydrogen for furfural reduction. However, the exogenous hydrogen system requires high initial hydrogen pressure, and the storage and use of hydrogen pose safety risks, making the operation extremely demanding. Previous studies often used precious metals as catalysts, demonstrating significant catalytic effects; however, the catalysts are costly to synthesize and complex, hindering their practical application. The catalytic conversion of furfural to levulinic ester involves multiple reaction stages, including reduction-alcoholization, each requiring different reaction conditions. Therefore, some studies have used multi-component catalysts to progressively convert furfural to levulinic ester through cascade reactions. However, this process is cumbersome, prone to side reactions, and the products are difficult to separate. Summary of the Invention

[0004] To overcome the defects and shortcomings of the existing technology, the purpose of this invention is to provide a substituted heteropolyacid catalyst, its preparation method and application.

[0005] The technical solution provided by this invention is as follows: A method for preparing a substituted heteropolyacid catalyst, the method comprising the following steps: (1) H3PW 12 O 40 Dissolve KCl in ultrapure water under magnetic stirring, and mix well to obtain a suspension. Adjust the pH of the suspension to 5.5, filter to remove insoluble matter, and dry the filtrate to obtain the vacancy-type potassium heteropolyacid K7PW.11 O 39 ; (2) The K7PW 11 O 39 Completely dissolved in ultrapure water, 0.035 g / mL cobalt chloride solution was added dropwise at 80°C with stirring until the reaction was complete. After cooling, the insoluble matter in the reaction product was removed by filtration. The resulting filtrate was quickly poured into a mixed solution of methanol and ethanol in a volume ratio of 1:1, and stirring was continued until precipitation was complete. The precipitate was collected by filtration, washed, dried, ground, and collected to obtain the substituted heteropolyacid potassium salt K5PW. 11 CoO 39 ; (3) The K5PW 11 CoO 39 The potassium ions were redissolved in ultrapure water and eluted with a 732 hydrogen-form strong acid cation exchange resin to replace hydrogen ions with potassium ions. The resulting solution was dried, ground, and collected to obtain the substituted heteropolyacid catalyst H5PW. 11 CoO 39 .

[0006] Preferably, in step (1), adjusting the pH of the suspension to 5.5 specifically involves: slowly adding 1 mol / L of KHCO3 aqueous solution to the suspension under magnetic stirring to adjust the pH of the suspension to 5.5.

[0007] Preferably, in step (1), the drying is performed at 80°C for 24 hours.

[0008] Preferably, in step (2), the washing involves repeatedly washing the precipitate with methanol three times; the drying involves drying at 80°C.

[0009] Preferably, in step (3), the drying is performed at 60°C for 36 h.

[0010] This invention further discloses the substituted heteropolyacid catalyst H5PW prepared by the above method. 11 CoO 39 .

[0011] This invention further discloses the application of the above-mentioned substituted heteropolyacid catalyst in the catalytic synthesis of isopropyl levulinate from furfural.

[0012] This invention further discloses a method for synthesizing isopropyl levulinate from furfural using the above-mentioned substituted heteropolyacid catalyst, the method comprising the following steps: 10-25 mg of the above-mentioned H5PW 11 CoO 393-6 mL of isopropanol and 10-40 mg of furfural were added sequentially to a polytetrafluoroethylene reactor. The reactor was sealed, and the mixture was stirred at 600 rpm and reacted at 140-170°C for 1-8 h. The reactor was then rapidly cooled to stop the reaction. The reaction mixture was centrifuged, and the supernatant was collected to obtain isopropyl levulinate.

[0013] Preferably, 20 mg of the H5PW 11 CoO 39 5 mL of isopropanol and 20 mg of furfural were added sequentially to a polytetrafluoroethylene reactor. The reactor was sealed, and the mixture was stirred at 600 rpm and reacted at 170°C for 4 h. The reactor was then rapidly cooled to stop the reaction. The reaction mixture was centrifuged, and the supernatant was collected to obtain isopropyl levulinate.

[0014] This invention overcomes the shortcomings of existing technologies by providing a substituted heteropolyacid catalyst, its preparation method, and its application. This invention introduces inexpensive transition metals to prepare a substituted dual-acid heteropolyacid catalyst with both Lewis and Brønsted acids, and tunable acidity, for catalyzing the conversion of furfural to isopropyl levulinate. Heteropolyacids are a class of polyacid compounds composed of transition metal-oxygen clusters (such as Mo, W) and central heteroatoms (such as P, Si). Due to their tunable Brønsted and Lewis acidity, reversible redox properties, and excellent electron transfer capabilities, they exhibit unique advantages in heterogeneous catalysis. In the furfural-catalyzed hydrogen transfer reaction, heteropolyacids often exhibit strong and singular Brønsted acidity, making them unsuitable for the multi-step cascade reaction of furfural. To address this, the present invention introduces other metal elements into the structure of Keggin-type heteropolyacids to adjust the structure, acidity, and redox properties of the heteropolyacids, thereby preparing multifunctional dual-acid heteropolyacid catalysts. These catalysts possess tunable Lewis and Brønsted acids, increasing the active sites of the heteropolyacids and effectively improving their catalytic conversion activity for furfural, thus enabling the highly selective synthesis of isopropyl levulinate.

[0015] The beneficial effects of this invention after adopting the above technical solution are as follows: (1) This invention uses catalytic hydrogen transfer technology, with inexpensive and stable isopropanol as solvent and hydrogen donor, to realize the conversion of furfural to isopropyl levulinate under mild conditions in a one-pot system, which simplifies the operation process; catalytic transfer hydrogenation is a green hydrogenation technology that replaces high-pressure hydrogen with hydrogen donors (such as alcohols), which significantly improves the safety of operation. It transfers hydrogen atoms from the hydrogen donor to the target substrate without introducing an external hydrogen source, and has the advantages of economy, low energy consumption, non-toxicity and high stability; (2) The substituted heteropolyacid catalyst H5PW used in this invention 11 CoO 39With a suitable ratio of Lewis and Brønsted acids, as well as good redox capabilities, it can efficiently promote furfural transfer and synthesize isopropyl levulinate under mild conditions with high selectivity, without the need to separate intermediate products in the reaction system. This simplifies the process, aligns with the development direction of green chemistry, and provides a highly promising catalytic pathway for the high-value utilization of biomass. Attached Figure Description

[0016] Figure 1 The substituted heteropolyacid H5PW prepared in this invention 11 CoO 39 H5PW 11 ZnO 39 H4PW 11 FeO 39 H4PW 11 AlO 39 Catalyst and commercially purchased H3PW 12 O 40 XRD diffraction pattern; Figure 2 The substituted heteropolyacid H5PW prepared in this invention 11 CoO 39 H5PW 11 ZnO 39 H4PW 11 FeO 39 H4PW 11 AlO 39 Catalyst and commercially purchased H3PW 12 O 40 The FTIR infrared spectrum. Detailed Implementation

[0017] The technical solution of the present invention will be further described in detail below with reference to specific embodiments, but this does not constitute any limitation on the present invention.

[0018] Examples 1-4 (1) Accurately weigh 18.0 g of H3PW 12 O 40 1.0 g of KCl was dissolved in 100 mL of ultrapure water under magnetic stirring. The solution was stirred continuously at room temperature for approximately 30 min to ensure complete precipitation. Approximately 40 mL of a 1 mol / L KHCO3 aqueous solution was slowly added dropwise to the suspension under magnetic stirring to adjust the pH to 5.5. After filtering to remove insoluble matter at room temperature, the filtrate was dried at 80 °C for 24 h to obtain a white solid, thus preparing the vacancy-type potassium heteropolyacid (K7PW). 11 O 39 ).

[0019] (2) Weigh 5.0 g of the prepared K7PW 11 O 39 The solid was added to a round-bottom flask containing 30 mL of ultrapure water and heated in a silicone oil pan at 80 °C with stirring until completely dissolved. Under magnetic stirring, a certain amount of cobalt chloride solution (0.175 g CoCl2 dissolved in 5 mL of water) was slowly added dropwise to the round-bottom flask. The mixture was heated and stirred for another 30 min. After cooling, the solution was filtered to remove insoluble matter. The filtrate was quickly poured into a beaker containing a 100 mL mixture of methanol and ethanol (volume ratio 1:1). A large amount of precipitate appeared. The mixture was stirred until precipitation was complete. The precipitate was collected by filtration and then washed three times with methanol. The resulting solid was dried at 80 °C, ground, and collected to obtain the substituted heteropolyacid potassium salt (K5PW). 11 CoO 39 ).

[0020] (3) K5PW 11 CoO 39 Dissolved in ultrapure water, potassium ions were replaced with hydrogen ions by elution using 732 hydrogen-type strong acid cation exchange resin until potassium ions were almost undetectable in the eluent. The replaced solution was then dried at 60°C for 36 h, ground, and collected to obtain the substituted heteropolyacid catalyst H5PW. 11 CoO 39 .

[0021] (4) Add 20 mg of H5PW sequentially to a 15 mL polytetrafluoroethylene reactor. 11 CoO 39 5 mL of isopropanol and 20 mg of furfural were added to a sealed reaction vessel. The mixture was stirred at 600 rpm and heated to 140℃, 150℃, 160℃, and 170℃ respectively, and maintained for 8 h. After heating, the reaction vessel was rapidly cooled to stop the reaction. The reaction mixture was centrifuged, and the supernatant was used to prepare standard solutions of furfural, furfuryl alcohol, and isopropyl levulinate. Quantitative analysis was performed using gas chromatography, and qualitative analysis was performed using gas chromatography-mass spectrometry. The results are listed in Table 1, numbers 1 to 4.

[0022] The effect of reaction temperature on catalytic activity was investigated. At 140℃ for 8 h, the furfural conversion rate was 87.0%, and the isopropyl levulinate yield was 63.2%. With increasing reaction temperature, both the furfural conversion rate and the isopropyl levulinate yield gradually increased. At 170℃ for 8 h, the furfural conversion rate reached 94.0%, and the isopropyl levulinate yield increased to 80.0%. This indicates that increasing the reaction temperature promotes the formation of the target product; therefore, the optimal reaction temperature is 170℃.

[0023] Examples 5-8 The corresponding substituted heteropolyacid catalyst H5PW was prepared according to the methods in Examples 1-4. 11 CoO 39 spare.

[0024] Add 20 mg of H5PW sequentially to a 15 mL polytetrafluoroethylene reactor. 11 CoO 39 5 mL of isopropanol and 20 mg of furfural were added to a sealed reaction vessel. The mixture was stirred at 600 rpm and heated to 170 °C, which was maintained for 1 h, 2 h, 4 h, and 6 h, respectively. After heating, the reaction vessel was rapidly cooled to stop the reaction. The reaction mixture was centrifuged, and the supernatant was used to prepare standard solutions of furfural, furfuryl alcohol, and isopropyl acetylpropionate. Quantitative analysis was performed using gas chromatography, and qualitative analysis was performed using gas chromatography-mass spectrometry. The results are listed in numbers 5 to 8 in Table 1.

[0025] The effect of reaction time on catalytic activity was investigated. At 170℃ for 1 h, the furfural conversion rate was 59.2% and the isopropyl levulinate yield was 50.0%. At 170℃ for 4 h, the furfural conversion rate was 83.2% and the isopropyl levulinate yield was 78.5%. Both the furfural conversion rate and the isopropyl levulinate yield increased significantly with increasing reaction time. However, at 170℃ for 6 h, the furfural conversion rate was 89.3% and the isopropyl levulinate yield was 79.0%. Based on Example 4, it was found that after heating for 4 h, the furfural conversion rate and the isopropyl levulinate yield did not increase significantly with increasing reaction time. Therefore, the optimal reaction time was 4 h.

[0026] Examples 9-11 The corresponding substituted heteropolyacid catalyst H5PW was prepared according to the methods in Examples 1-4. 11 CoO 39 spare.

[0027] Add 20 mg of H5PW to a 15 mL polytetrafluoroethylene reactor. 11 CoO 39 Add 10 mg, 30 mg and 40 mg of furfural to 5 mL of isopropanol, respectively. Seal the reaction vessel, stir at 600 rpm, heat to 170℃ and maintain for 4 h. After heating, rapidly cool the reaction vessel to stop the reaction. Centrifuge the reaction mixture, take the supernatant, and prepare standard solutions of furfural, furfuryl alcohol and isopropyl levulinate. Perform quantitative analysis using gas chromatography and qualitative analysis using gas chromatography-mass spectrometry. The results are listed in numbers 9 to 11 in Table 1.

[0028] The effect of furfural concentration on catalytic activity was investigated. At 170℃ and 4 h, with a furfural dosage of 10 mg, the furfural conversion rate was 86.1%, and the isopropyl levulinate yield was 42.5%. Referring to Example 7, when the furfural dosage was 20 mg, the furfural conversion rate increased to 83.2%, and the isopropyl levulinate yield increased to 78.5%. When the furfural dosage increased to 40 mg, the furfural conversion rate increased to 92.8%, but the isopropyl levulinate yield decreased significantly to only 38.2%. This is because when the furfural concentration is low, the intermediate products generated from furfural conversion are less likely to contact the active sites of the catalyst, resulting in a low isopropyl levulinate yield. Furfural is prone to polymerization, so when the concentration of furfural in the reaction system is too high, furfural polymerizes to form oligomer byproducts, which cannot be converted into intermediate products and isopropyl levulinate, thus leading to a decrease in the yield of isopropyl levulinate. Therefore, the optimal furfural dosage is 20 mg.

[0029] Examples 12-14 The corresponding substituted heteropolyacid catalyst H5PW was prepared according to the methods in Examples 1-4. 11 CoO 39 spare.

[0030] 10 mg, 15 mg, and 25 mg of H5PW were added to a 15 mL polytetrafluoroethylene reactor, respectively. 11 CoO 39 5 mL of isopropanol and 20 mg of furfural were added to a sealed reaction vessel. The mixture was stirred at 600 rpm and heated to 170 °C for 4 h. After heating, the reaction vessel was rapidly cooled to stop the reaction. The reaction mixture was centrifuged, and the supernatant was used to prepare standard solutions of furfural, furfuryl alcohol, and isopropyl levulinate. Quantitative analysis was performed using gas chromatography, and qualitative analysis was performed using gas chromatography-mass spectrometry. The results are listed in Table 1, numbers 12-14.

[0031] The catalyst H5PW was investigated. 11 CoO 39The effect of catalyst dosage on catalytic activity: Under the conditions of 170℃ and 4 h, when the catalyst dosage was 10 mg, the furfural conversion rate was 76.2% and the isopropyl levulinate yield was 56.3%; when the catalyst dosage was 15 mg, the furfural conversion rate was 82.0% and the isopropyl levulinate yield was 66.1%; however, when the catalyst dosage was further increased to 25 mg, the furfural conversion rate was 83.4% and the isopropyl levulinate yield was 71.0%. The furfural yield did not increase significantly, but the isopropyl levulinate yield decreased. This is because with the increase of catalyst, the acidic sites in the reaction system increase, which can promote the conversion of furfural to isopropyl levulinate. However, excessive acidity can lead to side reactions, generating oligomer complexes, thereby causing a decrease in the isopropyl levulinate yield. Based on Example 7, the optimal catalyst dosage is 20 mg.

[0032] Examples 15-17 The corresponding substituted heteropolyacid catalyst H5PW was prepared according to the methods in Examples 1-4. 11 CoO 39 spare.

[0033] Add 20 mg of H5PW to a 15 mL polytetrafluoroethylene reactor. 11 CoO 39 A certain amount of isopropanol and 20 mg of furfural were added. The volumes of isopropanol were 3 mL, 4 mL, and 6 mL, respectively. The reaction vessel was sealed, stirred at 600 rpm, and heated to 170°C. o After heating for 4 hours, the reaction vessel was rapidly cooled to stop the reaction. The reaction mixture was centrifuged, and the supernatant was collected to prepare standard solutions of furfural, furfuryl alcohol, and isopropyl acetylpropionate. Quantitative analysis was performed using gas chromatography, and qualitative analysis was performed using gas chromatography-mass spectrometry. The results are listed in numbers 15-17 in Table 1.

[0034] The effect of the amount of isopropanol on catalytic activity was investigated. At 170℃ and for 4 h, with 3 mL of isopropanol, the furfural conversion rate was 65.6% and the isopropyl levulinate yield was 57.9%. As shown in Example 7, the furfural conversion rate and isopropyl levulinate yield increased with increasing isopropanol volume. However, when the isopropanol volume was further increased to 6 mL, the furfural conversion rate was 83.6% and the isopropyl levulinate yield was 58.9%, indicating a stable furfural conversion rate but a decrease in the isopropyl levulinate yield. This is because a lower isopropanol volume resulted in a higher furfural concentration and acidity, making furfural more susceptible to side reactions and leading to lower furfural conversion and isopropyl levulinate yield. Conversely, excessive isopropanol diluted the reaction system, preventing effective contact between the intermediate product and the catalyst for further conversion into the target product, thus decreasing the isopropyl levulinate yield. Therefore, the optimal isopropanol volume was 5 mL.

[0035] Examples 18 and 19 20 mg H3PW was added to a 15 mL polytetrafluoroethylene reactor. 12 O 40 HY zeolite, 5 mL isopropanol and 20 mg furfural were added to a sealed reaction vessel. The mixture was stirred at 600 rpm and heated to 170 °C for 4 h. After heating, the reaction vessel was rapidly cooled to stop the reaction. The reaction mixture was centrifuged, and the supernatant was used to prepare standard solutions of furfural, furfuryl alcohol and isopropyl levulinate. Quantitative analysis was performed using gas chromatography and qualitative analysis was performed using gas chromatography-mass spectrometry. The results are listed in serial numbers 18 and 19 of Table 1.

[0036] The catalyst H5PW was investigated. 11 CoO 39 The effect of Lewis acid and Brønsted acid on catalytic activity. Under conditions of 170℃ and 4h, H3PW was used. 12 O 40 When used as a catalyst, furfural conversion was 73.5% and isopropyl levulinate yield was 53.7%; when using HY zeolite as a catalyst, furfural conversion was 86.2% and isopropyl levulinate yield was 21.6%; combined with Example 7, it was found that the prepared substituted heteropolyacid catalyst H5PW 11 CoO 39 Its catalytic effect is significantly better than that of H3PW. 12 O 40 And HY zeolite. In a one-pot system, the gradual conversion of furfural into isopropyl levulinate via a cascade reaction involves multiple reductive-alcoholic reactions, each requiring different catalytic conditions. H3PW 12 O 40Having a single Brønsted acid, HY zeolite has a single Lewis acid, so when using H3PW... 12 O 40 When HY zeolite is used as a catalyst, the reaction system relies on a single Brønsted or Lewis acid for catalysis, which is insufficient to meet the catalytic conditions for each stage of the furfural cascade reaction, thus failing to synthesize isopropyl levulinate with high selectivity. However, using H5PW... 11 CoO 39 When used as a catalyst, the metallic Co in the catalyst can provide Lewis acid. The Lewis acid and Brønsted acid in the reaction system synergistically catalyze the reaction, which can effectively improve the conversion rate of furfural and the yield of isopropyl levulinate.

[0037] Examples 20-22 The substituted heteropolyacid H5PW was prepared according to the methods in Examples 1-4. 11 ZnO 39 H4PW 11 FeO 39 and H4PW 11 AlO 39 Catalyst for later use.

[0038] I. Preparation of substituted heteropolyacid H5PW 11 ZnO 39 catalyst.

[0039] (1) Accurately weigh 18.0 g of H3PW 12 O 40 1.0 g of KCl was dissolved in 100 mL of ultrapure water under magnetic stirring. The solution was stirred continuously at room temperature for approximately 30 min to ensure complete precipitation. Approximately 40 mL of a 1 mol / L KHCO3 aqueous solution was slowly added dropwise to the suspension under magnetic stirring to adjust the pH to 5.5. After filtering to remove insoluble matter at room temperature, the filtrate was dried at 80 °C for 24 h to obtain a white solid, thus preparing the vacancy-type potassium heteropolyacid (K7PW). 11 O 39 ).

[0040] (2) Weigh 5.0 g of the prepared K7PW 11 O 39The solid was added to a round-bottom flask containing 30 mL of ultrapure water and heated in a silicone oil pan at 80 °C with stirring until completely dissolved. Under magnetic stirring, a certain amount of zinc chloride solution (0.184 g ZnCl2 dissolved in 5 mL of water) was slowly added dropwise to the round-bottom flask. The mixture was heated and stirred for another 30 min. After cooling, the solution was filtered to remove insoluble matter. The filtrate was quickly poured into a beaker containing 100 mL of a mixture of methanol and ethanol (volume ratio 1:1). A large amount of precipitate appeared. The mixture was stirred until precipitation was complete. The precipitate was collected by filtration and then washed three times with methanol. The resulting solid was dried at 80 °C, ground, and collected to obtain the substituted heteropolyacid potassium salt (K5PW). 11 ZnO 39 ).

[0041] (3) K5PW 11 ZnO 39 Dissolved in ultrapure water, potassium ions were replaced with hydrogen ions by elution using 732 hydrogen-type strong acid cation exchange resin until potassium ions were almost undetectable in the eluent. The replaced solution was then dried at 60°C for 36 h, ground, and collected to obtain the substituted heteropolyacid catalyst H5PW. 11 ZnO 39 .

[0042] II. Preparation of Substituted Heteropolyacid H4PW 11 FeO 39 catalyst.

[0043] (1) Accurately weigh 18.0 g of H3PW 12 O 40 1.0 g of KCl was dissolved in 100 mL of ultrapure water under magnetic stirring. The solution was stirred continuously at room temperature for approximately 30 min to ensure complete precipitation. Approximately 40 mL of a 1 mol / L KHCO3 aqueous solution was slowly added dropwise to the suspension under magnetic stirring to adjust the pH to 5.5. After filtering to remove insoluble matter at room temperature, the filtrate was dried at 80 °C for 24 h to obtain a white solid, thus preparing the vacancy-type potassium heteropolyacid (K7PW). 11 O 39 ).

[0044] (2) Weigh 5.0 g of the prepared K7PW 11 O 39The solid was added to a round-bottom flask containing 30 mL of ultrapure water and heated in a silicone oil pan at 80 °C with stirring until completely dissolved. Under magnetic stirring, a certain amount of ferric chloride solution (0.219 g FeCl3 dissolved in 5 mL of water) was slowly added dropwise to the round-bottom flask. The mixture was heated and stirred for another 30 min. After cooling, the solution was filtered to remove insoluble matter. The filtrate was quickly poured into a beaker containing 100 mL of a mixture of methanol and ethanol (volume ratio 1:1). A large amount of precipitate appeared. The mixture was stirred until precipitation was complete. The precipitate was collected by filtration and then washed three times with methanol. The resulting solid was dried at 80 °C, ground, and collected to obtain the substituted heteropolyacid potassium salt (K4PW). 11 FeO 39 ).

[0045] (3) K4PW 11 FeO 39 Dissolved in ultrapure water, potassium ions were replaced with hydrogen ions by elution using 732 hydrogen-type strong acid cation exchange resin until potassium ions were almost undetectable in the eluent. The replaced solution was then dried at 60°C for 36 h, ground, and collected to obtain the substituted heteropolyacid catalyst H4PW. 11 FeO 39 .

[0046] III. Preparation of Substituted Heteropolyacid H4PW 11 AlO 39 catalyst.

[0047] (1) Accurately weigh 18.0 g of H3PW 12 O 40 1.0 g of KCl was dissolved in 100 mL of ultrapure water under magnetic stirring. The solution was stirred continuously at room temperature for approximately 30 min to ensure complete precipitation. Approximately 40 mL of a 1 mol / L KHCO3 aqueous solution was slowly added dropwise to the suspension under magnetic stirring to adjust the pH to 5.5. After filtering to remove insoluble matter at room temperature, the filtrate was dried at 80 °C for 24 h to obtain a white solid, thus preparing the vacancy-type potassium heteropolyacid (K7PW). 11 O 39 ).

[0048] (2) Weigh 5.0 g of the prepared K7PW 11 O 39The solid was added to a round-bottom flask containing 30 mL of ultrapure water and heated in a silicone oil pan at 80 °C with stirring until completely dissolved. Under magnetic stirring, a certain amount of aluminum chloride solution (0.180 g AlCl3 dissolved in 5 mL of water) was slowly added dropwise to the round-bottom flask. The mixture was heated and stirred for another 30 min. After cooling, the solution was filtered to remove insoluble matter. The filtrate was quickly poured into a beaker containing 100 mL of a mixture of methanol and ethanol (volume ratio 1:1). A large amount of precipitate appeared. The mixture was stirred until precipitation was complete. The precipitate was collected by filtration and then washed three times with methanol. The resulting solid was dried at 80 °C, ground, and collected to obtain the substituted heteropolyacid potassium salt (K4PW). 11 AlO 39 ).

[0049] (3) K4PW 11 AlO 39 Dissolved in ultrapure water, potassium ions were replaced with hydrogen ions by elution using 732 hydrogen-type strong acid cation exchange resin until potassium ions were almost undetectable in the eluent. The replaced solution was then dried at 60°C for 36 h, ground, and collected to obtain the substituted heteropolyacid catalyst H4PW. 11 AlO 39 .

[0050] IV. Add 20 mg of H5PW to each 15 mL polytetrafluoroethylene reaction vessel. 11 ZnO 39 H4PW 11 FeO 39 H4PW 11 AlO 39 5 mL of isopropanol and 20 mg of furfural were added to a sealed reaction vessel. The mixture was stirred at 600 rpm and heated to 170 °C for 4 h. After heating, the reaction vessel was rapidly cooled to stop the reaction. The reaction mixture was centrifuged, and the supernatant was collected to prepare standard solutions of furfural, furfuryl alcohol, and isopropyl levulinate. Quantitative analysis was performed using gas chromatography, and qualitative analysis was performed using gas chromatography-mass spectrometry. The results are listed in Table 1, numbers 20-22.

[0051] Effect Example 1. Structural and crystallinity analysis of substituted heteropolyacids XRD analysis was performed on the substituted heteropolyacids H5PW prepared in Examples 1-4. 11 CoO 39 H5PW prepared in Examples 20-22 11 ZnO 39 H4PW 11 FeO 39 H4PW 11 AlO39 Catalyst and commercially purchased H3PW 12 O 40 It was characterized. For example... Figure 1 As shown, commercially purchased H3PW 12 O 40 Distinctive characteristic peaks can be observed at positions such as 2θ = 10°, 20.6°, 25°, 34.5°, 37.6°, 53.2°, and 59.8°, corresponding to the (110), (220), (222), (332), (510), (550), and (651) crystal planes of the polyacid cluster, respectively. Figure 1 It can be seen that all prepared substituted heteropolyacids H5PW 11 CoO 39 H5PW 11 ZnO 39 H4PW 11 FeO 39 and H4PW 11 AlO 39 The catalysts all exhibited similar performance to commercially purchased H3PW. 12 O 40 The consistent XRD diffraction peaks indicate that they have the same Keggin-type cluster structure, further demonstrating that Keggin-type substituted heteropolyacid catalysts can be successfully prepared by atomic substitution.

[0052] 2. Structural analysis of substituted heteropolyacid catalysts The H5PW prepared in Examples 1-4 11 CoO 39 H5PW prepared in Examples 20-22 11 ZnO 39 H4PW 11 FeO 39 H4PW 11 AlO 39 H3PW purchased commercially 12 O 40 FTIR spectral characterization was performed, and the results are as follows: Figure 2 As shown. Commercially purchased H3PW 12 O 40 At 800~1000 cm -1 There are four Keggin characteristic absorption peaks, which are generated by PO a W=O d WO b -W and WO c The stretching vibrations caused by -W are located at 1080 cm⁻¹. -1 980 cm -1 891 cm -1and 808 cm -1 Substituted heteropolyacid H5PW 11 CoO 39 H5PW 11 ZnO 39 H4PW 11 FeO 39 and H4PW 11 AlO 39 The catalyst exhibits four identical characteristic absorption peaks in the aforementioned region. This demonstrates that the prepared heteropolyacid catalyst possesses a complete Keggin structural unit.

[0053] 3. Effects of different substituted metals on catalytic activity Under conditions of 170℃ and 4 hours, H5PW was used. 11 ZnO 39 When used as a catalyst, furfural conversion was 80.6%, and isopropyl levulinate yield was 61.2%; using H4PW 11 FeO 39 When used as a catalyst, furfural conversion was 67.0%, and isopropyl levulinate yield was 62.8%; using H4PW 11 AlO 39 When used as a catalyst, furfural conversion was 86.3%, and isopropyl levulinate yield was 61.7%. Combined with Example 7, it was found that H5PW in the prepared substituted heteropolyacid catalyst... 11 CoO 39 The H5PW catalyst exhibits the best catalytic performance, with superior furfural conversion and isopropyl levulinate yield compared to other substituted heteropolyacid catalysts. 11 CoO3 contains a suitable ratio of Lewis acid and Brønsted acid, which can meet the catalytic conditions required for different reaction stages in the furfural cascade reaction, while H5PW 11 CoO3 has good redox properties and can promote the conversion of intermediate products to isopropyl levulinate, therefore H5PW 11 CoO 39 The catalyst exhibits the best catalytic effect.

[0054] 4. The conversion rates of furfural and the yields of isopropyl levulinate by the substituted heteropolyacid catalysts are shown in Table 1 below: Table 1 .

[0055] The above embodiments are preferred embodiments of the present invention, but the embodiments of the present invention are not limited to the above embodiments. Any changes, modifications, substitutions, combinations, or simplifications made without departing from the spirit and principle of the present invention shall be considered equivalent substitutions and shall be included within the protection scope of the present invention.

Claims

1. A method for preparing a substituted heteropolyacid catalyst, characterized in that, The method includes the following steps: (1) H3PW 12 O 40 Dissolve KCl in ultrapure water under magnetic stirring, and mix well to obtain a suspension. Adjust the pH of the suspension to 5.5, filter to remove insoluble matter, and dry the filtrate to obtain the vacancy-type potassium heteropolyacid K7PW. 11 O 39 ; (2) The K7PW 11 O 39 Completely dissolved in ultrapure water, 0.035 g / mL cobalt chloride solution was added dropwise at 80°C with stirring until the reaction was complete. After cooling, the insoluble matter in the reaction product was removed by filtration. The resulting filtrate was quickly poured into a mixed solution of methanol and ethanol in a volume ratio of 1:1, and stirring was continued until precipitation was complete. The precipitate was collected by filtration, washed, dried, ground, and collected to obtain the substituted heteropolyacid potassium salt K5PW. 11 CoO 39 ; (3) The K5PW 11 CoO 39 The potassium ions were redissolved in ultrapure water and eluted with a 732 hydrogen-form strong acid cation exchange resin to replace hydrogen ions with potassium ions. The resulting solution was dried, ground, and collected to obtain the substituted heteropolyacid catalyst H5PW. 11 CoO 39 .

2. The method as described in claim 1, characterized in that, In step (1), adjusting the pH of the suspension to 5.5 specifically involves slowly adding 1 mol / L of KHCO3 aqueous solution to the suspension under magnetic stirring to adjust the pH of the suspension to 5.

5.

3. The method as described in claim 1, characterized in that, In step (1), the drying is performed at 80°C for 24 hours.

4. The method as described in claim 1, characterized in that, In step (2), the washing involves repeatedly washing the precipitate with methanol three times; the drying involves drying at 80°C.

5. The method as described in claim 1, characterized in that, In step (3), the drying is performed at 60°C for 36 hours.

6. The substituted heteropolyacid catalyst H5PW prepared by the method according to any one of claims 1 to 5 11 CoO 39 .

7. The application of the substituted heteropolyacid catalyst of claim 6 in the catalytic synthesis of isopropyl levulinate from furfural.

8. A method for synthesizing isopropyl levulinate from furfural using the substituted heteropolyacid catalyst of claim 6, characterized in that, The method includes the following steps: taking 10-25 mg of the H5PW... 11 CoO 39 3-6 mL of isopropanol and 10-40 mg of furfural were added sequentially to a polytetrafluoroethylene reactor. The reactor was sealed, and the mixture was stirred at 600 rpm and reacted at 140-170°C for 1-8 h. The reactor was then rapidly cooled to stop the reaction. The reaction mixture was centrifuged, and the supernatant was collected to obtain isopropyl levulinate.

9. The method as described in claim 8, characterized in that, 20 mg of the H5PW 11 CoO 39 5 mL of isopropanol and 20 mg of furfural were added sequentially to a polytetrafluoroethylene reactor. The reactor was sealed, and the mixture was stirred at 600 rpm and reacted at 170°C for 4 h. The reactor was then rapidly cooled to stop the reaction. The reaction mixture was centrifuged, and the supernatant was collected to obtain isopropyl levulinate.

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