A magnetic sulfonated carbon nitride catalyst, its preparation and use in furanone production

Abstract

The application discloses a magnetic sulfonated carbon nitride catalyst and a preparation method and application thereof in furanone preparation. The preparation method comprises the following steps: sulfonated carbon nitride is obtained by reacting graphite phase carbon nitride with 1,3-propanesulfonic acid lactone under microwave assistance; carboxyl groups are introduced by reacting the sulfonated carbon nitride with succinic anhydride under microwave assistance; the sulfonated carbon nitride is covalently connected with amino-functionalized magnetic nanoparticles through amidation reaction; and finally, the sulfonic acid groups are partially neutralized by reacting the sulfonated carbon nitride with a part of alkali. The specific preparation sequence of "sulfonation-carboxyl group introduction-magnetic connection-final partial neutralization" is adopted, the exposure of the magnetic material in the organic sulfonation reaction is avoided, and the neutralization degree of the catalyst can be flexibly adjusted. Under the synergistic irradiation of microwaves and ultrasonic waves, the catalyst catalyzes the cyclization of 3,4-dihydroxy-2,5-hexanedione to prepare furanone, the reaction time is 45 min, the yield is 90.4%, the catalyst can be quickly recovered through magnetic separation, and the yield reaches 85% after the catalyst is recycled for 6 times.

Description

Technical Field

[0001] This invention belongs to the field of catalytic chemistry and organic synthesis technology, specifically relating to a magnetic core-shell structured sulfonated carbon nitride catalyst, its specific preparation method, and the application of this catalyst in the cyclization of 3,4-dihydroxy-2,5-hexanedione to furanone under synergistic microwave and ultrasonic irradiation. Background Technology

[0002] Furanone (4-hydroxy-2,5-dimethyl-3(2H)-furanone, commonly known as pineapple ketone) is a naturally occurring five-membered heterocyclic compound with a rich aroma of caramel, pineapple, and strawberry. Its aroma threshold is extremely low, approximately 0.04 ppb, making it one of the most important high-value-added flavorings in the food industry, widely used in beverages, confectionery, baked goods, and dairy products. Furthermore, furanone and its derivatives possess antibacterial and antioxidant bioactivities, finding wide applications in the pharmaceutical and daily chemical fields. Developing efficient, green, and industrially scalable furanone synthesis processes offers significant economic and social benefits.

[0003] Currently, the main synthetic routes for furanones include saccharide fermentation, tartrate ester cyclization, and acetone aldehyde cyclization. Among these, the acetone aldehyde cyclization route is the most competitive in industry due to its inexpensive raw materials and short steps. This route involves two steps: first, acetone aldehyde condensation to generate 3,4-dihydroxy-2,5-hexanedione; second, 3,4-dihydroxy-2,5-hexanedione cyclization to generate furanone. The second step, the cyclization reaction, is the key technical bottleneck of this route: the 3,4-dihydroxy-2,5-hexanedione molecule contains two ortho-hydroxyl groups and two ketone groups. The cyclization process involves intramolecular aldol condensation and dehydration, which places very stringent requirements on the base strength and steric selectivity of the catalyst. Without a catalyst or with an unsuitable catalyst, side reactions such as polymerization and resinification easily occur, leading to a decrease in furanone yield.

[0004] Existing catalytic systems have many shortcomings. Homogeneous base catalysts (such as Na₂HPO₄, Na₂CO₃, K₂CO₃, etc.), although inexpensive and readily available, are difficult to separate from the product, requiring steps such as neutralization, extraction, and distillation, resulting in poor atom economy and the generation of large amounts of waste salt and waste liquid. Solid base catalysts (such as hydrotalcite, MgO, CaO, etc.) applied to the cyclization reaction of 3,4-dihydroxy-2,5-hexanedione suffer from insufficient activity, poor selectivity, easy deactivation, limited mass transfer, and difficult recovery, with the number of recycling cycles generally not exceeding three.

[0005] Existing technologies share the following common shortcomings in three dimensions: catalyst design, preparation methods, and energy matching: outdated catalyst design concepts and a lack of recyclable and tunable multifunctional catalysts; the application of two-dimensional material-based solid bases in cyclization reactions is still in its infancy; the preparation sequence lacks innovation; the matching of energy sources is poor; and data on catalyst recovery and recycling are scarce. Summary of the Invention

[0006] This application provides a magnetic sulfonated carbon nitride catalyst, its preparation, and its application in the preparation of furanone. It develops a novel catalyst based on g-C3N4, which introduces controllable functional groups through covalent grafting and combines magnetic recovery and microwave-ultrasound synergistic energy to achieve efficient and green synthesis of furanone.

[0007] Graphitic carbon nitride (g-C3N4) possesses a unique two-dimensional layered structure, good thermal stability, and modifiability. Its abundant -NH2 groups at its edges can serve as anchors for covalent modification. However, research on using it as a platform for covalent modification, introducing controllable functional groups through chemical grafting, and its application in catalyzing the cyclization reaction of 3,4-dihydroxy-2,5-hexanedione has not yet been reported in the literature. Furthermore, conventional heating methods are time-consuming, energy-intensive, and prone to side reactions; while microwave heating alone can rapidly heat the bulk phase, its improvement on mass transfer in heterogeneous systems is limited; and ultrasonic heating alone is inefficient and difficult to achieve the reaction temperature on its own. When microwaves and ultrasound work synergistically, microwaves provide rapid and uniform dielectric heating, while ultrasound generates localized high temperature and pressure, microjets, and shock waves through cavitation effects, which can break up catalyst aggregates and enhance mass transfer at the solid-liquid interface, particularly beneficial for cyclization reactions involving solid catalysts. Therefore, the synergistic effect of these two methods holds promise for simultaneously solving the problems of heating efficiency and mass transfer limitations.

[0008] A method for preparing a magnetic sulfonated carbon nitride catalyst, comprising: (1) Graphite phase carbon nitride was reacted with 1,3-propanesulfonic acid lactone to obtain sulfonated carbon nitride g-C3N4-PrSO3H; (2) Sulfonated carbon nitride g-C3N4-PrSO3H is reacted with a carboxylating agent to introduce a carboxyl group, thereby obtaining carboxyl-modified sulfonated carbon nitride g-C3N4-PrSO3H-COOH; (3) The product g-C3N4-PrSO3H-COOH from step (2) is covalently linked to the aminated magnetic nanoparticles Fe3O4@SiO2-NH2 through an amidation reaction to obtain the magnetic linking intermediate Fe3O4@SiO2@g-C3N4-PrSO3H-COOH; (4) The product of step (3) is reacted with a partial base to neutralize the sulfonic acid group of the product of step (3) into a sulfonate group, thereby obtaining a magnetic sulfonated carbon nitride catalyst with both sulfonate group and sulfonic acid group acid-base bifunctional sites.

[0009] This catalyst uses graphitic carbon nitride (g-C3N4) as a matrix, introducing tunable sulfonic acid groups through covalent grafting of 1,3-propanesulfonic acid lactone, followed by modification with succinic anhydride to introduce carboxyl groups as magnetic anchors. These are then covalently linked to aminated magnetic nanoparticles via amide bonds. Finally, partial base neutralization forms a partially sodium sulfonate / sulfonic acid hybrid structure (acid-base bifunctional), resulting in a core-shell catalyst with both magnetic recovery capability and acid-base synergistic catalytic activity. This catalyst can be used for the cyclization of 3,4-dihydroxy-2,5-hexanedione to furanone under microwave-ultrasound synergistic irradiation.

[0010] The preparation method of this application uses microwave-assisted technology to accelerate the sulfonation reaction and the carboxyl group introduction reaction, achieves covalent connection between the magnetic core and the catalytic shell through the amidation reaction, and performs partial neutralization in the last step, thereby avoiding the exposure of magnetic materials in the organic sulfonation reaction, and at the same time achieving flexible adjustment of the catalyst neutralization degree.

[0011] Several alternative methods are provided below, but they are not intended as additional limitations on the overall solution above. They are merely further additions or optimizations. Provided there are no technical or logical contradictions, each alternative method can be combined individually with respect to the overall solution above, or multiple alternative methods can be combined with each other.

[0012] In this application, the term "partial base" refers to a base that is insufficient to completely convert all sulfonic acid groups (-SO3H) of the product in step (3) into sulfonate groups (e.g., sodium sulfonate-SO3Na), so that the catalyst surface simultaneously retains sulfonic acid groups (-SO3H) and sulfonate groups (e.g., -SO3Na), forming acid-base bifunctional sites.

[0013] Optionally, in step (4), 40% to 70% of the total sulfonic acid groups (-SO3H) in the product of step (3) are neutralized to sulfonate groups (e.g., -SO3Na). Further, 60% to 70% of the total sulfonic acid groups (-SO3H) in the product of step (3) are neutralized to sulfonate groups (e.g., -SO3Na).

[0014] The degree of partial neutralization can be precisely achieved by controlling the endpoint pH value of the alkali solution addition. Optionally, the endpoint pH value is controlled at 8.5~9.0, under which only a portion of the sulfonic acid groups are neutralized (60%~70%); complete neutralization requires adjusting the pH to a higher level. Specifically, the method for achieving the partial alkali effect is as follows: titrate with dilute alkali solution until the endpoint pH is controlled within an appropriate range (e.g., 8.5~9.0).

[0015] Furthermore, the base is NaOH (sulfonic acid group (-SO3H) is neutralized to sodium sulfonate group (-SO3Na)), the base concentration is 0.05~0.5 M, the reaction temperature is room temperature, and the reaction time is 1~4 h.

[0016] The methods for characterizing "partial neutralization" in the catalyst include, but are not limited to, any one or more of the following: (a) Back titration method: Weigh a certain amount of catalyst, add excess standard NaOH solution, and then titrate with standard HCl solution to determine the total sulfonic acid content; take another sample, exchange with NaCl and titrate to determine the -SO3H content; the difference between the two is the -SO3Na content, and the degree of neutralization can be calculated from this.

[0017] (b) X-ray photoelectron spectroscopy (XPS): Peak fitting was performed on the S 2p spectrum. The peak at 168.5±0.2 eV was assigned to -SO3H, and the peak at 167.2±0.2 eV was assigned to -SO3Na, confirming the coexistence of the two groups. The degree of neutralization was calculated by the ratio of their peak areas.

[0018] (c) Zeta potential analysis showed a significant shift in the isoelectric point, indicating a change in the surface charge properties.

[0019] Optionally, in step (1), the molar ratio of the graphitic carbon nitride to 1,3-propanesulfonic acid lactone is 1:1 to 5. Further optionally, the molar ratio is 1:3.

[0020] In step (2): Optionally, the carboxylating agent is succinic anhydride.

[0021] Optionally, the molar ratio of the carboxylating agent to the sulfonated carbon nitride g-C3N4-PrSO3H is 4.5~5.5:1, more preferably 5:1.

[0022] In step (3): Optionally, the aminated magnetic nanoparticles are Fe3O4@SiO2-NH2.

[0023] Optionally, the amidation reaction is carried out in the presence of EDC / NHS, and the ratio of g-C3N4-PrSO3H-COOH to EDC / NHS is 1:14~16:14~16 (preferably 1:15:15) based on the molar ratio of carboxyl group of g-C3N4-PrSO3H-COOH to EDC / NHS.

[0024] Optionally, the mass ratio of the aminated magnetic nanoparticles to g-C3N4-PrSO3H-COOH is 1.5~2.5:5; more preferably 2:5.

[0025] Optionally, the amidation reaction is carried out at a temperature of 20-50°C for 1-6 hours.

[0026] In step (4): Optionally, the conditions for the partial alkali reaction are as follows: the alkali is NaOH, the alkali concentration is 0.05~0.5 M, the reaction temperature is room temperature, the reaction time is 1~4 h, and the amount of alkali used is based on the final pH of the reaction system being 8.5~9.0.

[0027] Alternatively, the alkali concentration can be 0.1 M, the endpoint pH can be 8.5–9.0, and the time can be 2 h.

[0028] Optionally, steps (1), (2) and / or (3) are performed with microwave assistance. Preferably, steps (1), (2) and (3) are all performed with microwave assistance.

[0029] Optionally, the conditions for the microwave-assisted reaction in step (1) are: microwave power 100~400 W, temperature 50~80℃, and reaction time 0.5~3 h; further optionally, microwave power 200 W, temperature 70℃, and reaction time 1 h.

[0030] Optionally, the conditions for the microwave-assisted reaction in step (2) are: microwave power 200~400 W, temperature 60~100℃, and reaction time 30~90 min; further optionally, microwave power 350 W, temperature 80℃, and reaction time 45 min.

[0031] Optionally, the conditions for the microwave-assisted reaction in step (3) are: microwave power 200~400 W, reaction temperature 30~45℃, and reaction time 2~4 h; further optionally, microwave power 300 W, reaction temperature 40℃, and reaction time 3 h.

[0032] This application also provides a magnetic sulfonated carbon nitride catalyst prepared by the aforementioned preparation method.

[0033] The catalyst prepared in this application has a magnetic core-shell structure: the magnetic core is Fe3O4@SiO2-NH2, and the catalytically active shell is graphitic carbon nitride (g-C3N4-PrSO3Na / PrSO3H) covalently grafted with and partially neutralized from 1,3-propanesulfonic acid lactone, with the two covalently linked by amide bonds. This catalyst possesses both acid-base bifunctional sites (partial sodium sulfonate / sulfonic acid mixed structure), can be rapidly separated and recovered using an external magnetic field, and exhibits good recyclability.

[0034] This application also provides the application of the magnetic sulfonated carbon nitride catalyst in the catalytic synthesis of furanones.

[0035] This application also provides a method for preparing furanone, using 3,4-dihydroxy-2,5-hexanedione as a raw material and the magnetic sulfonated carbon nitride catalyst as a catalyst, to carry out a cyclization reaction under microwave and ultrasonic irradiation to generate furanone.

[0036] Optionally, the 3,4-dihydroxy-2,5-hexanedione is prepared by acetone-aldehyde condensation.

[0037] The synthetic route for furanones is as follows: Step 1: Acetone-aldehyde condensation to form 3,4-dihydroxy-2,5-hexanedione 2 CH3COCHO → CH3CO-CHOH-CHOH-COCH3; Step 2: Cyclization of 3,4-dihydroxy-2,5-hexanedione to form furanone (catalytic step in this application) CH3CO-CHOH-CHOH-COCH3→ C6H8O3+ H2O.

[0038] Optionally, in the cyclization reaction, the power of the microwave is 100~500 W, the power of the ultrasound is 100~500 W, the reaction temperature is 30~80℃, and the reaction time is 15~90 min.

[0039] Optionally, the catalyst amount is 2 to 8% of the mass of 3,4-dihydroxy-2,5-hexanedione. More optionally, the catalyst amount is 3% to 5%.

[0040] Optionally, the solvent for the cyclization reaction is anhydrous ethanol, and the cyclization reaction is carried out under N2 protection.

[0041] Optionally, after the reaction is complete, the catalyst is recovered by an external magnetic field, washed and dried with deionized water and ethanol, and then used directly in the next batch of reaction.

[0042] In the most preferred embodiment, the catalyst dosage is 5% of the substrate mass, the microwave power is 400 W, the ultrasonic power is 300 W, the reaction temperature is 70℃, and the reaction time is 45 min. Under these conditions, the furanone separation yield can reach 90.4%.

[0043] Compared with the prior art, this application has at least one of the following beneficial effects: (1) This invention employs a specific preparation sequence of sulfonation-carboxyl group introduction-magnetic linkage-final partial neutralization, placing the neutralization step after magnetic linkage to avoid the exposure of magnetic materials in the organic sulfonation reaction. Simultaneously, the degree of catalyst neutralization can be flexibly adjusted as needed. Comparative experiments demonstrate that the success of this sequence requires creative effort, and partial neutralization (around 60%) is key to obtaining high cyclization activity.

[0044] (2) The present invention uses microwave assistance in the two steps of sulfonation and carboxyl introduction, which significantly shortens the reaction time: the sulfonation reaction is shortened from 6 h of conventional heating to 1 h; the carboxyl introduction reaction is shortened from 6 h to 45 min; and the amidation reaction is shortened from 24 h of conventional heating to 3 h.

[0045] (3) The catalyst prepared in this invention has both acid-base bifunctional sites (partial sodium sulfonate / sulfonic acid mixed structure). Under microwave-ultrasound synergistic irradiation, it catalyzes the cyclization of 3,4-dihydroxy-2,5-hexanedione to furanone in only 45 min with a yield of 90.4%. Moreover, the yield of microwave-ultrasound synergistic irradiation is significantly higher than the sum of the yields of microwave alone and ultrasound alone, proving that there is a synergistic effect, which is especially beneficial to cyclization reactions involving solid catalysts.

[0046] (4) The catalyst of the present invention can be quickly recovered through magnetic separation. After being recycled 6 times, the yield remains >85%, which is economical and suitable for continuous production. Attached Figure Description

[0047] Figure 1 The image shows the FTIR spectrum of the catalyst in Example 1 (where curve a is g-C3N4-PrSO3H, curve b is Fe3O4, curve c is Fe3O4@SiO2@g-C3N4-PrSO3H, and curve d is the final catalyst in Example 1). Detailed Implementation

[0048] The technical solution of this application will be clearly and completely described below with reference to the embodiments of this application. Obviously, the described embodiments are only some embodiments of this application, and not all embodiments. Based on the embodiments of this application, all other embodiments obtained by those of ordinary skill in the art without creative effort are within the scope of protection of this application.

[0049] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this application belongs. The terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of the application.

[0050] Unless otherwise stated, the ethanol used in the specific embodiments of the present invention is anhydrous ethanol, and the raw materials and reagents used are as shown in the figure and are all commercially available products.

[0051] The aminated magnetic nanoparticles used in the following examples are Fe3O4@SiO2-NH2, and the preparation process is as follows: (1) Dissolve 2.0 g of FeCl2·4H2O and 5.4 g of FeCl3·6H2O in 100 mL of deionized water (N2 protection), add NH3·H2O dropwise under vigorous stirring until pH=10, age at 80℃ for 1 h, perform magnetic separation, wash 3 times with deionized water to obtain Fe3O4.

[0052] (2) Disperse Fe3O4 in 200 mL of ethanol / water (4:1, v / v), add 5 mL of concentrated ammonia and 2 mL of TEOS, stir at room temperature for 12 h, perform magnetic separation, wash with ethanol 3 times to obtain Fe3O4@SiO2.

[0053] (3) Disperse Fe3O4@SiO2 in 100 mL of anhydrous ethanol, add 2 mL of APTES, reflux for 6 h under N2 protection, perform magnetic separation, wash 3 times with anhydrous ethanol, and vacuum dry at 60 °C to obtain Fe3O4@SiO2-NH2.

[0054] Example 1 (1) Sulfonation 20 g of urea was placed in a covered crucible and thermally polymerized in a muffle furnace at 550 °C for 4 h at a heating rate of 5 °C / min. After natural cooling to room temperature, 3.2 g of g-C3N4 was obtained.

[0055] 1.0 g (10.9 mmol) of g-C3N4 was dispersed in 30 mL of anhydrous acetonitrile and ultrasonically dispersed for 15 min. 32.7 mmol of 1,3-propanesulfonic acid lactone was added, and the mixture was transferred to a microwave reaction tube. The reaction was carried out at 70 °C and 200 W for 1 h. After cooling, the mixture was centrifuged and washed successively with acetonitrile, ethanol, and deionized water. The mixture was then vacuum dried at 60 °C for 12 h to obtain g-C3N4-PrSO3H.

[0056] (2) Introduction of carboxyl groups Take 1.0 g of the sulfonation product g-C3N4-PrSO3H, add 2.33 g of succinic anhydride (molar ratio of succinic anhydride to g-C3N4-PrSO3H 5:1), 0.03 g of triethylamine, and 20 mL of anhydrous DMF. Transfer to a microwave reaction tube, and react for 45 min under N2 protection at 350 W microwave power and 80 °C. After cooling, centrifuge and wash successively with DMF, ethanol, and deionized water. Dry under vacuum at 60 °C for 12 h to obtain g-C3N4-PrSO3H-COOH. Acid-base titration: carboxyl content 0.63 mmol / g.

[0057] (3) Magnetic connection 0.5 g of the product g-C3N4-PrSO3H-COOH from the carboxyl introduction step was dispersed in MES buffer (0.1 M, pH 5.8–6.0, 20 mL). 0.91 g of EDC·HCl and 0.54 g of NHS (carboxyl:EDC:NHS molar ratio 1:15:15) were added, and the mixture was stirred at room temperature for 30 min. 0.2 g of Fe3O4@SiO2-NH2 was added, and the MES buffer was brought to a final volume of 40 mL. The mixture was transferred to a microwave reaction tube and reacted at 300 W at 40 °C for 3 h. After magnetic separation, the mixture was washed successively with MES buffer, deionized water, and ethanol, and then vacuum dried at 60 °C for 12 h to obtain the magnetically linked intermediate Fe3O4@SiO2@g-C3N4-PrSO3H-COOH.

[0058] (4) Partial neutralization 0.5 g of the product from the magnetic bonding step, Fe3O4@SiO2@g-C3N4-PrSO3H-COOH, was dispersed in 20 mL of deionized water. The solution was slowly titrated with 0.1 M NaOH solution under stirring until the final pH reached 8.5–9.0. Stirring was continued at room temperature for 2 h. After magnetic separation, the solution was washed with deionized water until neutral and dried to obtain the final catalyst Fe3O4@SiO2@g-C3N4-PrSO3Na / PrSO3H. Back titration determined the total sulfonic acid group content of the catalyst to be 0.61 mmol / g, with -SO3H content at 0.23 mmol / g and -SO3Na content at 0.38 mmol / g, resulting in a neutralization degree of approximately 62.3%.

[0059] The FTIR spectra of the products obtained in each step are as follows: Figure 1 As shown, curve a represents g-C3N4-PrSO3H, curve b represents Fe3O4, curve c represents Fe3O4@SiO2@g-C3N4-PrSO3H, and curve d represents the final catalyst of Example 1. The results show that curve a has a peak performance at 1040 cm⁻¹. -1 and 1170 cm -1 The presence of symmetric and asymmetric stretching vibration peaks at -S=O at point C proves successful grafting of the sulfonic acid group; curves c and d at 1650 cm⁻¹ -1 The presence of an absorption peak in the vicinity of the amide I band (C=O stretching) confirms the successful formation of the amide bond.

[0060] Example 2 Preparation of furanone (1) Add 40 mL of 25% acetone aldehyde and 10 mL of water to a four-necked flask, purge with nitrogen and start the stirrer. After 30 min, add 3 g of zinc powder and 2 g of phase transfer agent tetrabutylammonium iodide, control the temperature in a water bath to below 30 °C, and slowly add 75 mL of acetic acid solution (20% acetic acid). Then add the remaining zinc powder (total zinc powder 11.2 g) several times, maintaining the temperature inside the flask at 25-30 °C. After the addition is complete, continue stirring for 1 hour. Filter by suction, adjust the filtrate to neutral with 10% NaHCO3 solution, evaporate the solvent, extract with ethyl acetate (50 mL × 5), dry with anhydrous MgSO4, and rotary evaporate to obtain 12.5 g of 3,4-dihydroxy-2,5-hexanedione, yield 70%.

[0061] (2) 0.05 mol (7.3 g) of 3,4-dihydroxy-2,5-hexanedione, 100 mL of 95% ethanol, and 0.37 g (5 wt%) of the catalyst from Example 1 were added to a microwave-ultrasound synergistic reactor. The air was replaced three times with N2. The microwave power was set to 400 W, the ultrasonic power to 300 W, and the reaction was carried out at 70 °C for 45 min. After the reaction was complete, the catalyst was adsorbed using an external magnet (<1 min), and the reaction solution was decanted. The reaction solution was rotary evaporated to obtain the furanone product. GC analysis showed a conversion rate of 97.1% ± 1.1%, a furanone selectivity of 93.1% ± 0.9%, and a yield of 90.4% ± 1.3%.

[0062] Example 3 The reaction was carried out according to the method of Example 2. After the reaction in step (2) was completed, the catalyst was magnetically separated and recovered. After washing with deionized water and ethanol and drying, it was directly used for the next batch of reaction. The results of 6 cycles are shown in Table 1.

[0063] Table 1 Results of catalyst recycling

[0064] As shown in Table 1, the yield remained >85% after 6 cycles, and the magnetic separation operation time was <1 min.

[0065] Comparative Example 1 Catalysts were prepared using different preparation sequences and then subjected to cyclization reactions according to the method in Example 2. The results are shown in Table 2.

[0066] Table 2 Effect of preparation order on catalyst preparation and cyclization yield

[0067] The preparation route of catalyst B is to adjust the preparation steps of Example 1 to (1) sulfonation - (2) carboxyl group introduction - (3) partial neutralization - (4) magnetic connection, while keeping other conditions unchanged; the preparation route of catalyst C is to adjust the preparation steps of Example 1 to (1) sulfonation - (2) partial neutralization - (3) carboxyl group introduction - (4) magnetic connection, while keeping other conditions unchanged; the preparation route of catalyst D is to adjust the preparation steps of Example 1 to (1) magnetic connection - (2) sulfonation - (3) carboxyl group introduction - (4) partial neutralization, while keeping other conditions unchanged; the preparation route of catalyst E is to adjust the preparation steps of Example 1 to (1) sulfonation - (2) magnetic connection - (3) carboxyl group introduction - (4) partial neutralization, while keeping other conditions unchanged.

[0068] in: The failure of catalyst C is due to premature partial neutralization, which causes significant changes in the surface charge and polarity of g-C3N4-PrSO3Na after neutralization. This results in a substantial reduction in its contact efficiency with the electrophilic reagent succinic anhydride, preventing the effective introduction of the carboxyl group.

[0069] The reason for the failure of catalyst D is that the original g-C3N4 surface without any modification lacks functional groups that can form stable covalent bonds with amino magnetic particles (only a small amount of -NH2 at the edge, but the reactivity and grafting density are insufficient), which leads to the magnetic particles being mainly physically adsorbed and falling off during post-treatment. Furthermore, the exposed Fe3O4 is severely corroded and agglomerated in the subsequent sulfonation reaction (which includes strong acidity and high temperature).

[0070] The failure of catalyst E is due to the fact that magnetic bonding was performed before the introduction of the carboxyl group. The surface of g-C3N4-PrSO3H still lacked a -COOH group as a covalent anchor, and -NH2 could not form a stable amide bond with g-C3N4-PrSO3H, resulting in extremely low bonding efficiency. Subsequent carboxyl group introduction reaction conditions (high temperature, DMF) further disrupted the unstable physical adsorption structure.

[0071] As shown in Table 2, both Route A (this application) and Route B can be successfully prepared. Route A places neutralization last to avoid the exposure of magnetic materials in the organic sulfonation reaction, and the degree of neutralization can be flexibly adjusted as needed.

[0072] This application places the introduction of magnetic particles after all harsh organic reactions (sulfonation, carboxyl group introduction) to avoid Fe3O4 dissolving, agglomerating, or contaminating the catalyst surface under high-temperature conditions of strong organic acids. The core advantage of pre-linking followed by neutralization lies in protecting the amide bonds: if neutralization occurs before linking, the conversion of sulfonic acid groups to sodium sulfonate will change the molecular polarity and reactivity, reducing the efficiency of magnetic linking; post-neutralization ensures efficient amide bond formation and uses only dilute alkali for a short time to treat the magnetic particles, maximizing their integrity.

[0073] Comparative Example 2 The cyclization reaction was carried out according to the method in Example 2, and the performance of catalysts prepared at different neutralization times was compared. The results are shown in Table 3.

[0074] Table 3. Effects of neutralization timing on cyclization yield and cycle stability

[0075] Catalyst B was prepared in the same manner as Comparative Example 1. Catalyst F was prepared by removing the partial neutralization treatment in step (4) of Example 1, while the rest was the same as in Example 1. Catalyst G was prepared by replacing the partial neutralization in step (4) of Example 1 with complete neutralization. Specifically, 0.5 g of the magnetic linking intermediate Fe3O4@SiO2@g-C3N4-PrSO3H-COOH was dispersed in 20 mL of deionized water and slowly titrated with 0.1 M NaOH solution under stirring until the final pH of the system was 12.0~13.0. Stirring was continued at room temperature for 4 h to ensure complete neutralization. Magnetic separation was performed, and the catalyst was washed with deionized water until neutral and dried to obtain the completely neutralized catalyst Fe3O4@SiO2@g-C3N4-PrSO3Na. The content of -SO3H in the catalyst was 0 mmol / g, the content of -SO3Na was 0.61 mmol / g, and the degree of neutralization was 100% by back titration.

[0076] As shown in Table 3, partial neutralization (around 60%) is the key to obtaining high cyclization activity. The timing of neutralization (before or after ligation) and the degree of neutralization have a significant impact on the activity. Neutralization after ligation not only has a higher yield but also offers the advantage of process flexibility.

[0077] Comparative Example 3 The cyclization reaction was carried out according to the method of Example 2. Different energy sources were compared, and other aspects were the same as in Example 2. The results are shown in Table 4.

[0078] Table 4. Impact of different energy sources on ring-forming yield

[0079] As shown in Table 4, microwave The yield of ultrasound synergistic effect was significantly higher than the sum of the yields of microwave and ultrasound alone, demonstrating the existence of synergistic effect, which is particularly beneficial to mass transfer and heating in the cyclization reaction.

[0080] Comparative Example 4 The cyclization reaction was carried out according to the method of Example 2. Different catalysts were compared, and other reaction conditions were the same as in Example 2. The results are shown in Table 5.

[0081] Table 5 Catalytic performance of different catalysts for cyclization reaction

[0082] The results in Table 5 show that the catalyst prepared in this application has better catalytic activity and selectivity than similar products, and is easy to recover and recycle.

[0083] The embodiments described above are merely illustrative of several implementation methods of this application, and while the descriptions are relatively specific and detailed, they should not be construed as limiting the scope of the invention patent. It should be noted that those skilled in the art can make various modifications and improvements without departing from the concept of this application, and these all fall within the protection scope of this application. Therefore, the protection scope of this patent application should be determined by the appended claims.

Claims

1. A method for preparing a magnetic sulfonated carbon nitride catalyst, characterized in that, include: (1) Graphite phase carbon nitride was reacted with 1,3-propanesulfonic acid lactone to obtain sulfonated carbon nitride g-C3N4-PrSO3H; (2) Sulfonated carbon nitride g-C3N4-PrSO3H is reacted with a carboxylating agent to introduce a carboxyl group, thereby obtaining carboxyl-modified sulfonated carbon nitride g-C3N4-PrSO3H-COOH; (3) The product g-C3N4-PrSO3H-COOH from step (2) is covalently linked to the aminated magnetic nanoparticles Fe3O4@SiO2-NH2 through an amidation reaction to obtain the magnetic linking intermediate Fe3O4@SiO2@g-C3N4-PrSO3H-COOH; (4) The product of step (3) is reacted with a partial base to neutralize the sulfonic acid group of the product of step (3) into a sulfonate group, thereby obtaining a magnetic sulfonated carbon nitride catalyst with both sulfonate group and sulfonic acid group acid-base bifunctional sites.

2. The preparation method according to claim 1, characterized in that, In step (1), the molar ratio of the graphite phase carbon nitride to 1,3-propanesulfonic acid lactone is 1:1~5; in step (2), the carboxylating agent is succinic anhydride; the molar ratio of the carboxylating agent to sulfonated carbon nitride g-C3N4-PrSO3H is 4.5~5.5:

1.

3. The preparation method according to claim 1, characterized in that, In step (3): The amidation reaction is carried out in the presence of EDC / NHS, and the ratio of g-C3N4-PrSO3H-COOH to EDC / NHS is calculated as 1:14~16:14~16 of the molar ratio of carboxyl group of g-C3N4-PrSO3H-COOH to EDC / NHS. The mass ratio of the aminated magnetic nanoparticles Fe3O4@SiO2-NH2 to g-C3N4-PrSO3H-COOH is 1.5~2.5:

5.

4. The preparation method according to claim 1, characterized in that, In step (4), the sulfonic acid groups that account for 40% to 70% of the total sulfonic acid groups in the product of step (3) are neutralized into sulfonate groups.

5. The preparation method according to claim 4, characterized in that, In step (4), the amount of alkali used is based on the fact that the final pH of the reaction system is 8.5 to 9.

0.

6. The preparation method according to claim 5, characterized in that, The alkali is NaOH, with a concentration of 0.05~0.5 M, the reaction temperature is room temperature, and the reaction time is 1~4 h.

7. The preparation method according to claim 1, characterized in that, Steps (1), (2) and / or (3) are performed with microwave assistance; The conditions for microwave-assisted reaction in step (1) are: microwave power 100~400 W, temperature 50~80℃, and reaction time 0.5~3h; The conditions for microwave-assisted reaction in step (2) are: microwave power 200~400 W, temperature 60~100℃, and reaction time 30~90 min; The conditions for microwave-assisted reaction in step (3) are: microwave power 200~400 W, reaction temperature 30~45℃, and reaction time 2~4h.

8. A magnetic sulfonated carbon nitride catalyst prepared by the preparation method according to any one of claims 1 to 7.

9. A method for preparing furanone, characterized in that, Using 3,4-dihydroxy-2,5-hexanedione as a raw material and the magnetic sulfonated carbon nitride catalyst as described in claim 8 as a catalyst, a cyclization reaction is carried out under the combined irradiation of microwave and ultrasound to generate furanone.

10. The method according to claim 9, characterized in that, The 3,4-dihydroxy-2,5-hexanedione is prepared by acetone-aldehyde condensation; the microwave power is 100-500 W, the ultrasonic power is 100-500 W, the reaction temperature is 30-80℃, and the reaction time is 15-90 min; the catalyst dosage is 2-8% of the mass of 3,4-dihydroxy-2,5-hexanedione; the catalyst is recovered and recycled through an external magnetic field.

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