Method for preparing 2, 4-dinitroanisole through photo-thermal catalytic continuous nitration

Abstract

The invention belongs to the technical field of synthesis of fine chemical intermediates such as dyes, medicines, pesticides and explosives, and provides a method for preparing 2, 4-dinitroanisole through photo-thermal catalysis and continuous nitration. 2, 4-dinitroanisole is prepared in a continuous flow reactor through a two-stage nitration process, anisole and N-nitrosaccharin form mixed droplets in a bidirectional nozzle impinging stream mode, the mixed droplets enter the continuous flow reactor and react under visible light for 2-5 minutes, a nitration product is obtained through photo-thermal catalytic nitration reaction, the nitration product continuously enters a second-stage reactor, and a nitration product is obtained; the method comprises the following steps of: mixing the 2, 4-dinitroanisole with an acetonitrile solution of new N-nitrosaccharin in a continuous flow reactor, reacting for 2-5 minutes under visible light, and performing photo-thermal catalytic nitration reaction to obtain the main product 2, 4-dinitroanisole. According to the method, the reaction speed of N-nitrosaccharin and anisole can be increased in the continuous flow reactor, stable, safe and efficient synthesis of 2, 4-dinitroanisole can be achieved through the photo-thermal catalysis technology, and the product yield reaches 83%-91%.

Description

Technical Field

[0001] The present invention belongs to the technical field of the synthesis of fine chemical intermediates such as dyes, pharmaceuticals, pesticides, and explosives, and particularly relates to a method for the preparation of 2,4-dinitroanisole by photocatalytic continuous nitration; further, a two-stage photocatalytic nitration method for the preparation of 2,4-dinitroanisole in a continuous flow reactor is provided. Background Art

[0002] 2,4-Dinitroanisole (DNAN) is a classic military insensitive explosive [Cent. Eur. J. Energ. Mater. 2023, 20(1): 50-74]. The nitro group can be reduced or partially reduced to obtain 2,4-diaminoanisole, 2-amino-4-nitroanisole, or 4-amino-2-nitroanisole, which can be used to obtain dye, pigment, perfume, or pharmaceutical intermediates through diazotization, coupling, and acylation reactions [CN201910157994.0; CN202111386022.2; CN202311059803.X; CN202311820155.5], and even participate in the synthesis of polyamide products [CN202210390405.5]. 2,4-Dinitrophenol obtained by the hydrolysis of anisole can be converted into dye, pigment, perfume, pesticide, or pharmaceutical intermediates through acylation, esterification, condensation, oxidation, and other reactions [CN202310462696.9; CN202310826718.5; CN202410020169.7; CN202411306157.7].

[0003] It is known that its preparation process mostly involves corrosive strong acids and bases. For example, dinitroanisole is usually prepared by nitrating chlorobenzene with a mixed acid of nitric acid and sulfuric acid, and then carrying out etherification with sodium methoxide (or methanol + sodium hydroxide) [Energetic Materials, 2020, 28(1): 13-24]. This not only poses serious safety risks but also may cause harm to operators and the environment. Therefore, it is particularly important to develop a green synthesis process for dinitroanisole.

[0004] The introduction of adiabatic continuous flow reaction technology into the nitration industry effectively improves the synthesis efficiency and safety factor of the nitration process after splitting nitric acid, sulfuric acid, etc. into tiny reaction units. This is because compared with traditional reactors, microreactors have a smaller volume, can effectively reduce the liquid holdup, and at the same time, due to their narrow heat exchange channels, the risk of thermal runaway is effectively avoided. However, the continued use of hazardous chemicals still fails to ensure the true inherent safety of the entire process. Therefore, researchers are working hard to develop safer and greener nitrating reagents to replace traditional nitric acid reagents. For example, easily dissociable N / O / S-nitroheterocyclic compounds, as a new type of nitrating reagent [Synthesis 2020, 52, 796-806; Org. Lett. 2020, 22, 2714-2719; JACS Au 2022, 2, 2152-2161], provide a new approach for the nitration of aromatic hydrocarbons.

[0005] In traditional nitration processes, the introduction of corrosion-resistant catalysts can significantly improve the reaction efficiency and reaction conditions. For example, heterogeneous catalysts such as heteropolyacids and solid acids can participate in the nitration reaction of NO2(g) and are particularly suitable for multiphase continuous flow reactors with solid fillers [CN202410292814.0]. The characteristics of homogeneous catalysts such as metal-organic complexes and metal-organic acid salts enable full-radiation photocatalytic nitration synthesis or partial-radiation photothermal / photoelectrocatalytic synthesis to greatly reduce the reaction energy barrier and reaction energy consumption, which is a green and efficient synthesis strategy. However, at present, due to the high cost of catalysts and difficulties in separation and recovery, it has not yet met the industrial requirements for continuous production. Non-metallic carbon nitride photocatalysts can also effectively promote the nitration reaction due to their good corrosion resistance, such as the liquid-phase nitration of pyrrole and phenol [Sustain. Chem. Pharm., 2023, 33: 101077; J. Photochem. Photobiol. A: Chem., 2024, 456: 115796]. However, this heterogeneous photocatalyst has certain limitations when directly used in liquid-solid continuous flow reactors, such as being prone to clogging pipelines, which may cause safety accidents. Therefore, although non-metallic carbon nitride photocatalysts have certain application potential in batch production, their application in continuous flow reactors still requires further technical improvement and optimization.

[0006] As a material with a unique electronic structure, carbon nitride (C3N4) has received extensive attention due to its potential applications in fields such as photothermal / photoelectrocatalysis. Dispersing and loading carbon nitride on different continuous carriers, such as glass, ceramics, resin, or carbon fiber, can form a stable catalyst, which may be used in continuous production processes. Due to chemical properties, the combination of carbon nitride and carbon-based carriers is particularly economical and effective. In the existing literature, the commonly used method is to attach a carbon nitride precursor, such as melamine, to a carbon-based material, or coexist melamine with a carbon precursor, and then obtain a composite photocatalyst of C3N4 and a carbon-based material through high-temperature calcination. The process preparation is cumbersome and energy consumption is relatively high. In contrast, the direct coating process has lower energy consumption and is easy to operate, but it is unstable itself and prone to loss. Summary of the Invention

[0007] The present invention provides a method for continuously preparing 2,4-dinitroanisole by photothermal catalysis. Further, 2,4-dinitroanisole is prepared by two-stage photothermal catalytic nitration in a continuous flow reactor.

[0008] The present invention is realized by the following technical solutions: A method for continuously preparing 2,4-dinitroanisole by photothermal catalysis, in a continuous flow reactor, 2,4-dinitroanisole is prepared by a two-stage nitration process, specifically including the following steps: The first stage: controlling the molar ratio of anisole to N-nitrosaccharin to be 1:1.2 to 1:1.5, mixing a 20% wt. acetic acid solution of anisole and a 20% wt. acetonitrile solution of N-nitrosaccharin in a way of impinging flow of a two-way nozzle to form mixed small droplets, entering the continuous flow reactor, controlling the temperature in the reactor to be 25 - 45 °C, turning on the LED visible light source, and the residence reaction time is 2 - 5 minutes. Through photothermal catalytic nitration reaction, a mononitration product is obtained, and the yield is 96% - 100% wt. The mononitration product includes o-nitroanisole and p-nitroanisole, and their respective mass ratios are 74% - 92% wt. and 8% - 26% wt.; The second stage: the liquid from the first stage continuously enters the second reactor, mixes with a new 20% wt. acetonitrile solution of N-nitrosaccharin, enters the continuous flow reactor, controls the temperature in the reactor to be 45 - 65 °C, and the temperature in the second reactor > the temperature in the first reactor, turns on the LED visible light source, and the residence reaction time is 2 - 5 minutes. Through photothermal catalytic nitration reaction, the main product 2,4-dinitroanisole and a small amount of p-nitroanisole are obtained, and the yield of the main product 2,4-dinitroanisole is 83% - 91%; Among them: The continuous flow reactor is filled with fluorinated C3N4 / carbon fiber cloth as the catalyst for the photothermal nitration reaction. The preparation process is as follows: After cleaning 2-5 g of carbon fiber cloth, it is immersed in a solution of 100 mL of 25%-35% wt. nitric acid and 8-12 g of sodium fluoride, heated to 65-80 °C and maintained for 1-2 h, then 1-2.5 g of graphitic C3N4 is added, and the immersion is continued for 2-5 h. Then it is fished out, washed until neutral, and sprayed with a 3% wt. boron trifluoride etherate solution on the cloth as the chemically bonded catalyst. The dosage of the 3% wt. boron trifluoride etherate solution is 0.05~0.2 mL / cm 2 , and heat-treated in a nitrogen stream at 150~180 °C for 5~8 h to obtain the fluorinated C3N4 / carbon fiber catalyst.

[0009] The N-nitrosaccharin can be replaced by N-nitro lactam, N-nitro pyridine, N-nitro pyrazole or N-nitro imidazole.

[0010] The continuous flow reactor is externally provided with a plastic shell and heated with hot air to control the indoor temperature; it is internally provided with an LED light source with a power of 45-75 W and a peak wavelength of 390 nm~520 nm; the liquid flowing out at the end of the reaction is introduced into low-temperature ice water at 0~15 °C with a volume ratio of 1 / 10~1 / 20, so that the nitration product precipitates and settles as a solid and is collected regularly, while the acetic acid / acetonitrile waste liquid diluted by the ice water is recovered by distillation and concentration.

[0011] The C3N4 photocatalyst supported on carbon fiber cloth of the present invention is not only easy to operate, but also firmly bonds the two together through chemical bonds. Especially after fluorination treatment, the surface performance of the composite catalyst has been significantly improved, which greatly enhances its activity and stability in the catalytic process. At the same time, it also shows excellent high selectivity in the nitration reaction of anisole, which is in sharp contrast to the result that the para-nitration product of anisole dominates in previous studies. Traditionally, due to the electronic effect and steric effect of the methoxy group, the para-nitration product in the nitration product of anisole is often superior to the ortho-nitration product [J. Mol.Catal. A: Chem., 2015, 407: 81-86; CN201710070710.5; JACS Au 2022, 2: 2152−2161].

[0012] However, in the first-stage nitration reaction of the present invention, the selectivity of the formation of o-nitroanisole is unexpectedly higher than that of p-nitroanisole. In the second-stage nitration reaction, o-nitroanisole shows an even higher conversion rate than p-nitroanisole, and thus is more easily converted into 2,4-dinitroanisole. This novel catalyst and the continuous flow photothermal catalytic nitration process greatly improve the selectivity and efficiency of the nitration reaction, indicating its application prospect in the synthesis technology field of fine chemical intermediates such as dinitroanisole.

[0013] In the present invention: Carbon fiber has good thermal conductivity and can transfer reaction heat in a timely manner. After being treated with nitric acid and sodium fluoride together, carboxylation and hydroxylation modification occur on the surface, and then chemical bonding is formed with C3N4. At the same time, the hydrogen bond interaction between the catalyst surface and anisole is promoted, which is beneficial to the adsorption and activation of the substrate; protonated C3N4 has good photocatalytic nitration ability. After fluorination treatment, it also participates in capturing photogenerated electrons and causes hole enrichment, and the holes just act with nitrogen oxides to facilitate the formation of NO2 + nitration intermediate. The photothermal nitration reaction catalyst composed of the two has obvious stability and catalytic ability, and can accelerate the reaction rate of N-nitrosaccharin and anisole in a continuous flow reactor. Through this process, the stable, safe and efficient synthesis of 2,4-dinitroanisole can be realized, and the product yield reaches 83% - 91%. Description of the Drawings

[0014] Figure 1 SEM photographs of carbon fiber cloth (a) and carbon fiber cloth growing C3N4 (b); Figure 2 FTIR spectra of carbon fiber cloth and carbon fiber cloth growing C3N4; Figure 3 XPS full spectra of carbon fiber cloth and carbon fiber cloth growing C3N4; Figure 4 1H NMR nuclear magnetic and FTIR infrared spectra of 2,4-dinitroanisole; In the figure: (a) is the 1H NMR nuclear magnetic spectrum; (b) is the FTIR infrared spectrum. Detailed Embodiments

[0015] To make the objectives, technical solutions and advantages of the embodiments of the present invention clearer, the technical solutions in the embodiments of the present invention will be clearly and completely described below. Obviously, the described embodiments are part of the embodiments of the present invention, rather than all of the embodiments; based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative efforts shall fall within the protection scope of the present invention.

[0016] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by those skilled in the art to which the present invention belongs. The materials cited herein and their cited materials will be incorporated by reference.

[0017] Equivalent technologies of the specific embodiments described that can be understood by those skilled in the art through routine experiments will be included in this application.

[0018] In the experimental methods of the following examples, unless otherwise specified, they are all conventional methods. The instruments and equipment used in the following examples, unless otherwise specified, are all conventional laboratory instruments and equipment; the experimental materials used in the following examples, unless otherwise specified, are all obtained from conventional biochemical reagent stores.

[0019] Example 1: A method for the continuous photothermal catalytic nitration to prepare 2,4-dinitroanisole, the specific steps are as follows: In a continuous flow reactor, 2,4-dinitroanisole is prepared by a two-stage nitration process: The first stage: Control the molar ratio of anisole to N-nitrosaccharin to be 1:1.2. A 20% wt. acetic acid solution of anisole and a 20% wt. acetonitrile solution of N-nitrosaccharin are formed into mixed small droplets in a two-way nozzle impinging stream manner and enter the continuous flow reactor. The indoor temperature is controlled at 25 °C and the LED visible light source is turned on. The residence reaction time is 5 minutes. The nitration product is obtained by photothermal catalytic nitration reaction, and the yield is 96% wt., including o-nitroanisole and p-nitroanisole, and their respective mass ratios are 92% wt. and 8% wt. The second stage: The liquid from the first stage continuously enters the second reactor, is mixed similarly with a new 20% wt. acetonitrile solution of N-nitrosaccharin, enters the continuous flow reactor. The indoor temperature is controlled at 45 °C and the LED visible light source is turned on. The residence reaction time is 5 minutes. The main product 2,4-dinitroanisole and a small amount of p-nitroanisole are obtained by photothermal catalytic nitration reaction, and the yield of the main product is 91%.

[0020] In the above continuous nitration process, N-nitrosaccharin can also be replaced by N-nitro lactam, N-nitropyridine, N-nitropyrazole or N-nitroimidazole, etc.

[0021] The above continuous flow reactor is filled with fluorinated C3N4 / carbon fiber cloth, which is a catalyst for photothermal nitration reaction. The preparation process is as follows: After washing 5 g of carbon fiber cloth, it is immersed in a solution of 100 mL of 35% wt. nitric acid and 12 g of sodium fluoride, heated to 65 °C and maintained for 1-2 h, then 2.5 g of graphitic C3N4 is added, and the immersion is continued for 2 h. After fishing out and washing to neutrality, a 3% wt. boron trifluoride diethyl ether solution is sprayed on the cloth as a chemically bonded catalyst (the dosage is 0.2 mL / cm 2 ), and heat treatment is carried out in a nitrogen stream at 180 °C for 5 h to obtain a fluorinated C3N4 / carbon fiber catalyst. The SEM electron micrograph of the obtained C3N4 / carbon fiber cloth is as Figure 1 shown.

[0022] In the figure: (a) is the unmodified carbon fiber cloth with a relatively smooth surface, and (b) is the carbon fiber cloth composite with C3N4, whose surface is significantly rough, proving the successful loading of catalyst particles on the carbon fiber cloth.

[0023] The continuous flow reactor has an external plastic shell heated by hot air to control the indoor temperature; an internal LED light source with a power of 45 W and a peak wavelength of 390 nm; the liquid flowing out at the end of the reaction is introduced into ice water at 0 °C (volume ratio 1 / 20) to precipitate and settle the nitrification product as a solid, and it is collected regularly. The acetic acid / acetonitrile waste liquid diluted by ice water can be recovered by distillation and concentration.

[0024] Example 2: In a continuous flow reactor, 2,4-dinitroanisole was prepared by a two-stage nitrification process: The first stage: Control the molar ratio of anisole to N-nitro lactam to be 1:1.3. A 20% wt. acetic acid solution of anisole and a 20% wt. acetonitrile solution of N-nitro lactam are in the form of a collision flow of a two-way nozzle to form mixed small droplets, which enter the continuous flow reactor. The indoor temperature is controlled at 35 °C and the LED visible light source is turned on. The residence reaction time is 4 minutes. The mononitration product is obtained by a photothermal catalytic nitration reaction with a yield of 97% wt., including o-nitroanisole and p-nitroanisole, and their respective mass ratios are 88% wt. and 12% wt. The second stage: The liquid from the first stage continuously enters the second reactor, is mixed similarly with a new 20% wt. acetonitrile solution of N-nitro lactam, enters the continuous flow reactor. The indoor temperature is controlled at 50 °C and the LED visible light source is turned on. The residence reaction time is 4 minutes. The 2,4-dinitroanisole and a small amount of p-nitroanisole are obtained by a photothermal catalytic nitration reaction, and the yield of the main product is 87%.

[0025] The above continuous flow reactor is filled with fluorinated C3N4 / carbon fiber cloth, which is a catalyst for photothermal nitration reaction. Its preparation process is as follows: After cleaning 4 g of carbon fiber cloth, it is immersed in a solution of 100 mL of 30% wt. nitric acid and 10 g of sodium fluoride, heated to 70 °C and maintained for 1 - 2 h, then 2 g of graphitic C3N4 is added, and it is continuously boiled for 3 h. It is fished out and washed to neutrality, and a 3% wt. boron trifluoride diethyl ether solution is sprayed on the cloth as a chemically bonded catalyst (the dosage is 0.15 mL / cm 2 ), and it is heat-treated in a nitrogen stream at 170 °C for 6 h to obtain a fluorinated C3N4 / carbon fiber catalyst. The infrared spectrum of the obtained C3N4 / carbon fiber cloth is as Figure 2 shown.

[0026] In the figure: The red line is the carbon fiber cloth composite with C3N4, and the black line is the unmodified carbon fiber cloth. The surface of the unmodified carbon fiber cloth contains aldehyde and ketone C=O (496 cm -1, the in-plane bending vibration attributed to C-C=O) and the carboxyl C=O (1792 cm -1 ), the strong characteristic peak at 1414 cm -1 is attributed to the O=C-O of the carboxyl group. After reacting with C3N4, this characteristic peak disappears or weakens, and instead, the strong characteristic peak is at 1385 cm -1 , which should be attributed to the O=C-N vibration of the amide. The characteristic peak of -OH is around 3300 cm -1 . At the same time, it is worth noting that on the black line, the strong absorption peak at 729 cm -1 may be attributed to the out-of-plane bending vibration of O-H, and the weak absorption peak at 1384 cm -1 is attributed to the in-plane bending vibration of O-H. When transitioning from the black line to the red line, these peaks all weaken, and these changes further illustrate that the carboxylic acid on the carbon fiber surface reacts chemically with C3N4 to form an amide bond complex.

[0027] Continuous flow reactor, equipped with an LED light source inside, with a power of 50 W and a peak wavelength of 420 nm; the liquid flowing out at the end of the reaction is introduced into 5°C low-temperature ice water (volume ratio 1 / 15) to precipitate and settle the nitrification product as a solid, and collect it regularly, while the acetic acid / acetonitrile waste liquid diluted by the ice water can be recovered by distillation and concentration.

[0028] Example 3: In a continuous flow reactor, 2,4-dinitroanisole was prepared by a two-stage nitrification process: First stage: Control the molar ratio of anisole to N-nitropyridine to be 1:1.4. A 20% wt. acetic acid solution of anisole and a 20% wt. acetonitrile solution of N-nitropyridine are in the form of a two-way nozzle impinging stream to form mixed small droplets, which enter the continuous flow reactor. The indoor temperature is controlled at 40°C and the LED visible light source is turned on. The residence reaction time is 3 minutes, and the mononitration product is obtained by photo-thermal catalytic nitrification reaction, with a yield of 98% wt., including o-nitroanisole and p-nitroanisole, and their respective mass ratios are 80% wt. and 20% wt.; Second stage: The liquid from the first stage continuously enters the second reactor, undergoes a similar mixing with a new 20% wt. acetonitrile solution of N-nitropyridine, enters the continuous flow reactor. The indoor temperature is controlled at 60°C and the LED visible light source is turned on. The residence reaction time is 3 minutes, and 2,4-dinitroanisole and a small amount of p-nitroanisole are obtained by photo-thermal catalytic nitrification reaction, with the main product yield being 85%.

[0029] The above continuous flow reactor is filled with fluorinated C3N4 / carbon fiber cloth, which is a catalyst for photothermal nitration reaction. The preparation process is as follows: After cleaning 3 g of carbon fiber cloth, it is immersed in a solution of 100 mL of 27% wt. nitric acid and 9 g of sodium fluoride, heated to 75 °C and maintained for 1 - 2 h. Then, 1.5 g of graphitic C3N4 is added, and the mixture is continuously boiled for 4 h. After fishing out and washing until neutral, a 3% wt. boron trifluoride diethyl ether solution is sprayed on the cloth as a chemically bonded catalyst (the dosage is 0.1 mL / cm 2 ), and heat treatment is carried out in a nitrogen stream at 160 °C for 7 h to obtain a fluorinated C3N4 / carbon fiber catalyst. The XPS spectrum of the obtained C3N4 / carbon fiber cloth is as shown in Figure 3 .

[0030] In the figure: The red line is the carbon fiber cloth composite C3N4, and the black line is the unmodified carbon fiber cloth. The surface non-metal element distribution after fluorination and oxidation of the carbon fiber cloth can clearly confirm the presence of fluorine, carbon, carbon, nitrogen, etc. The ratio of the number of nitrogen atoms to the number of carbon atoms in the unmodified carbon fiber cloth is only 0.03. After the carbon fiber cloth reacts chemically with C3N4 to form a composite material, this nitrogen-carbon atom ratio rapidly increases to 0.33, and the characteristic peak of N1s is significantly enhanced, proving that a large amount of nitrogen elements are introduced onto the surface of the carbon fiber cloth due to the composite of C3N4.

[0031] The continuous flow reactor is internally equipped with an LED light source with a power of 60 W and a peak wavelength of 460 nm. The liquid flowing out at the end of the reaction is introduced into 10 °C low-temperature ice water (volume ratio 1 / 12) to precipitate and settle the nitration product as a solid, which is collected regularly. The acetic acid / acetonitrile waste liquid diluted by the ice water can be recovered by distillation and concentration.

[0032] Example 4: In a continuous flow reactor, 2,4-dinitroanisole is prepared by a two-stage nitration process: The first stage: Control the molar ratio of anisole to N-nitropyrazole to be 1:1.5. A 20% wt. acetic acid solution of anisole and a 20% wt. acetonitrile solution of N-nitropyrazole are formed into mixed small droplets in a way of impinging flow with a two-way nozzle and enter the continuous flow reactor. The indoor temperature is controlled at 45 °C and the LED visible light source is turned on. The residence reaction time is 2 minutes, and a mononitration product is obtained by photothermal catalytic nitration reaction with a yield of 100% wt., including o-nitroanisole and p-nitroanisole, and their respective mass ratios are 74% wt. and 26% wt.; The second stage: The liquid from the first stage continuously enters the second-stage reactor, undergoes a similar mixing with a new acetonitrile solution of 20% wt. N-nitropyrazole, enters a continuous-flow reactor, the indoor temperature is controlled at 65 °C and an LED visible light source is turned on, the residence reaction time is 2 minutes, and 2,4-dinitroanisole and a small amount of p-nitroanisole are obtained through a photothermal catalytic nitration reaction, with the yield of the main product being 83%.

[0033] The continuous-flow reactor is filled with fluorinated C3N4 / carbon fiber cloth, which is the catalyst for the photothermal nitration reaction: The preparation process is as follows. After cleaning, 2 g of carbon fiber cloth is immersed in a solution of 100 mL of 25% wt. nitric acid and 8 g of sodium fluoride, heated to 80 °C and maintained for 1 - 2 h, then 1 g of graphitic C3N4 is added, and the immersion is continued for 5 h. After fishing out and washing until neutral, a 3% wt. boron trifluoride diethyl ether solution is sprayed on the cloth as the chemically bonded catalyst (the dosage is 0.05 mL / cm 2 ), and it is heat-treated in a nitrogen stream at 150 °C for 8 h to obtain the fluorinated C3N4 / carbon fiber catalyst.

[0034] The continuous-flow reactor is equipped with an internal LED light source with a power of 75 W and a peak wavelength of 520 nm; the liquid flowing out at the end of the reaction is introduced into 15 °C low-temperature ice water (volume ratio 1 / 10), so that the nitration products precipitate and settle as solids and are collected regularly, while the acetic acid / acetonitrile waste liquid diluted by the ice water can be recovered by distillation and concentration. The nuclear magnetic and infrared spectra of the obtained 2,4-dinitroanisole product are as Figure 4 shown.

[0035] In the figure: (a) is the nuclear magnetic resonance spectrum of 2,4-dinitroanisole, and H a , H b , H c , H d correspond to the methoxy group (-OCH3) on the structure of 2,4-dinitroanisole and the C-H at positions 2, 3, and 5 of the aromatic ring in sequence. (b) is the infrared spectrum of 2,4-dinitroanisole. The asymmetric stretching vibration and symmetric stretching vibration peaks of the nitro group (-NO2) appear near 1520 cm⁻¹ and 1340 cm⁻¹, the characteristic absorption peak of the methoxy group (-OCH3) appears at 1280 cm⁻¹, the asymmetric stretching vibration and symmetric stretching vibration peaks of the C-H of the methoxy group appear at 2950 cm⁻¹ and 2850 cm⁻¹, multiple medium-strong peaks appear in the range of 1450 - 1600 cm⁻¹ and are attributed to the stretching vibration of the benzene ring C=C, the ortho- and meta-position C-H bending vibration peaks of 2,4-dinitroanisole appear near 750 cm⁻¹ and 800 cm⁻¹, and the characteristic peaks in the range of the remaining 600 - 900 cm⁻¹ are attributed to the frequency conversion or combination frequency vibration peaks of the aromatic -NO2.

[0036] Comparative Example 1: The reaction was carried out using a common autoclave photocatalytic reactor. The C3N4 / carbon fiber cloth photocatalyst was the same as in Example 1. The molar ratio of anisole to N-nitrosaccharin was controlled to be 1:2.4. A 20%wt. acetic acid solution of anisole and a 20%wt. acetonitrile solution of N-nitrosaccharin were directly mixed. The temperature was controlled at 45 °C and an LED visible light source (power 45 W, peak wavelength 390 nm) was turned on. The stirring reaction time was 60 minutes. Among the obtained products, the yields of 2,4-dinitroanisole, o-nitroanisole, and p-nitroanisole were 52.9%, 27.5%, and 19.6% in sequence.

[0037] Comparative Example 2: The reaction was carried out using a common autoclave photocatalytic reactor. The C3N4 / carbon fiber cloth photocatalyst was the same as in Example 2. The molar ratio of anisole to N-nitrosaccharin was controlled to be 1:2.6. A 20%wt. acetic acid solution of anisole and a 20%wt. acetonitrile solution of N-nitrosaccharin were directly mixed. The temperature was controlled at 50 °C and an LED visible light source (power 50 W, peak wavelength 420 nm) was turned on. The stirring reaction time was 60 minutes. Among the obtained products, the yields of 2,4-dinitroanisole, o-nitroanisole, and p-nitroanisole were 46.5%, 32.3%, and 21.2% in sequence.

[0038] Comparative Example 3: The reaction was carried out using a common autoclave photocatalytic reactor. The C3N4 / carbon fiber cloth photocatalyst was the same as in Example 3. The molar ratio of anisole to N-nitrosaccharin was controlled to be 1:2.8. A 20%wt. acetic acid solution of anisole and a 20%wt. acetonitrile solution of N-nitrosaccharin were directly mixed. The temperature was controlled at 60 °C and an LED visible light source (power 60 W, peak wavelength 460 nm) was turned on. The stirring reaction time was 60 minutes. Among the obtained products, the yields of 2,4-dinitroanisole, o-nitroanisole, and p-nitroanisole were 44.7%, 30.5%, and 24.8% in sequence.

[0039] Comparative Example 4: The reaction was carried out using a common autoclave photocatalytic reactor. The C3N4 / carbon fiber cloth photocatalyst was the same as in Example 4. The molar ratio of anisole to N-nitrosaccharin was controlled to be 1:3. A 20%wt. acetic acid solution of anisole and a 20%wt. acetonitrile solution of N-nitrosaccharin were directly mixed. The temperature was controlled at 65 °C and an LED visible light source (power 75 W, peak wavelength 520 nm) was turned on. The stirring reaction time was 60 minutes. Among the obtained products, the yields of 2,4-dinitroanisole, o-nitroanisole, and p-nitroanisole were 44.9%, 31.4%, and 23.7% in sequence.

[0040] It can be seen from the results of the comparative examples and examples that the present invention can significantly improve the yield of the end product 2,4-dinitroanisole.

[0041] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, rather than to limit it; although the present invention has been described in detail with reference to the foregoing embodiments, those of ordinary skill in the art should understand that they can still modify the technical solutions described in the foregoing embodiments, or perform equivalent replacements on some or all of the technical features; and these modifications or replacements do not make the essence of the corresponding technical solutions deviate from the scope of the technical solutions of the embodiments of the present invention.

Claims

1. A method for continuously preparing 2,4-dinitroanisole by photothermal catalysis nitration, characterized in that: In a continuous flow reactor, dinitroanisole is prepared by a two-stage nitration process, which specifically includes the following steps: First stage: Control the molar ratio of anisole to N-nitrosaccharin to be 1:1.2 - 1:1.

5. A 20% wt. acetic acid solution of anisole and a 20% wt. acetonitrile solution of N-nitrosaccharin are formed into mixed small droplets in a way of impinging flow by a two-way nozzle and enter the continuous flow reactor. Control the temperature in the reactor to be 25 - 45°C, and turn on the LED visible light source. The residence reaction time is 2 - 5 minutes, and a mononitration product is obtained through photo-thermal catalytic nitration reaction. The yield is 96% - 100% wt. The mononitration product includes o-nitroanisole and p-nitroanisole, and their respective mass ratios are 74% - 92% wt. and 8% - 26% wt.; Second stage: The liquid from the first stage continuously enters the second-stage reactor, mixes with a new 20% wt. acetonitrile solution of N-nitrosaccharin, and enters the continuous flow reactor. Control the temperature in the reactor to be 45 - 65°C, and the temperature in the second-stage reactor > the temperature in the first-stage reactor. Turn on the LED visible light source, and the residence reaction time is 2 - 5 minutes. The main product 2,4-dinitroanisole and a small amount of p-nitroanisole are obtained through photo-thermal catalytic nitration reaction, and the yield of the main product 2,4-dinitroanisole is 83% - 91%; Among them: The continuous flow reactor is filled with fluorinated C3N4 / carbon fiber cloth as the catalyst for the photothermal nitration reaction. The preparation process is as follows: After cleaning 2-5 g of carbon fiber cloth, it is immersed in a solution of 100 mL of 25%-35% wt. nitric acid and 8-12 g of sodium fluoride, heated to 65-80 °C and maintained for 1-2 h. Then, 1-2.5 g of graphitic C3N4 is added, and the immersion is continued for 2-5 h. After fishing out and washing until neutral, a 3% wt. boron trifluoride etherate solution is sprayed on the cloth, and the dosage of the 3% wt. boron trifluoride etherate solution is 0.05-0.2 mL / cm 2 , and heat treatment is carried out in a nitrogen gas stream at 150-180 °C for 5-8 h to obtain the fluorinated C3N4 / carbon fiber catalyst.

2. The method according to claim 1, wherein: The N-nitrosaccharin can be replaced by N-nitro lactam, N-nitro pyridine, N-nitro pyrazole or N-nitro imidazole.

3. The method according to claim 1, wherein: The continuous flow reactor is externally provided with a plastic shell for heating with hot air and controlling the indoor temperature; internally provided with an LED light source with a power of 45 - 75 W and a peak wavelength of 390 nm - 520 nm. The liquid flowing out at the end of the reaction is introduced into low-temperature ice water at 0 - 15°C with a volume ratio of 1 / 10 - 1 / 20, so that the nitration product precipitates and settles as a solid and is collected regularly. The acetic acid / acetonitrile waste liquid diluted by the ice water is recovered by distillation and concentration.

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