A method for preparing nitromethane by continuous flow photocatalytic nitration of methane

Abstract

This invention provides a method for the continuous flow photocatalytic nitration of methane to nitromethane. The method uses nitric acid as the nitrating agent and, under light irradiation, utilizes a reaction system consisting of an iron-based photocatalyst, additives, and a solvent to achieve the continuous conversion of methane to nitromethane in a continuous flow microchannel reactor. The method of this invention features mild reaction conditions, can be safely carried out at room temperature and moderate pressure, and does not use highly toxic reagents. The iron-based catalyst used is low-cost, stable, and easily recyclable, and exhibits high product selectivity, with a nitromethane to methyl nitrate selectivity ratio exceeding 10:1, which facilitates product separation and purification, and has good prospects for industrial application.

Description

Technical Field

[0001] This invention belongs to the field of methane high-value utilization and green chemical process technology, specifically relating to a method for preparing nitromethane by continuous flow photocatalytic nitration of methane. Background Technology

[0002] Methane, a major component of natural gas, is abundant and widely available, serving as both an important clean energy source and a key C1 chemical feedstock. However, traditional methane utilization primarily relies on combustion for energy and high-temperature conversion, which generates substantial carbon dioxide emissions. Furthermore, the release of methane during extraction, transportation, and use contributes significantly to the greenhouse effect. Moreover, current mainstream routes for converting methane into chemicals (such as steam reforming to syngas and its downstream chemical conversions) typically depend on high-temperature, high-energy-consumption processes, resulting in high energy efficiency and carbon emission intensity. Therefore, developing advanced catalytic and process technologies capable of directly converting methane into high-value-added chemicals under milder conditions is crucial for reducing energy consumption, emissions, and promoting green and sustainable development.

[0003] Nitromethane is an important organic chemical raw material and organic synthesis intermediate. At room temperature, it is a colorless, transparent, oily liquid with a faint odor. It is widely used in polymer materials, energetic materials, rocket propellant systems, gasoline additives, coatings, and in the pharmaceutical, pesticide, and dye industries. Due to its high polarity and good solubility, it can also be used as a solvent. The current industrial production of nitromethane mainly relies on two traditional processes: (1) the replacement method of dimethyl sulfate and sodium nitrite: nitromethane is generated through nucleophilic substitution reaction at a relatively mild temperature. This method is relatively simple to operate, but it requires the use of highly toxic dimethyl sulfate and generates a large amount of sodium sulfate waste residue, which poses significant safety and environmental pressures and cost issues (e.g., patents CN102659602, CN103553924, etc.); (2) the gas-phase nitration method: based on the free radical chain reaction of alkanes and nitric acid under high temperature (350–450 ℃) and high pressure (0.8–1.2 MPa) conditions, the products are usually a mixture of various nitroalkanes (nitromethane, nitroethane, nitropropane, etc.), and the selectivity of nitromethane is low; at the same time, the high temperature and high pressure and the highly corrosive system put forward higher requirements for equipment and operation safety (e.g., patents CN102574770, CN104003885, etc.).

[0004] In 2023, Professor Wang Cheng's research group at Fudan University reported a photocatalyzed aerobic nitration reaction of methane: in an aqueous system at ambient pressure and approximately 100°C, driven by 430 nm light and using aluminum nitrate as the nitrating agent, they successfully achieved the conversion of methane to methyl nitrate (CH3ONO2) with a selectivity of 78%, while nitromethane (CH3NO2) as a byproduct had a selectivity of 9%. However, this process only achieved a high selectivity for the conversion of methyl nitrate, while the selectivity for nitromethane was extremely low, and the catalytic efficiency and reaction rate were insufficient to meet industrial requirements.

[0005] Therefore, there is an urgent need in this field to develop a new green process that can efficiently and selectively convert methane into nitromethane under mild conditions and is suitable for continuous production, which has important technological value and industrial significance. Summary of the Invention

[0006] The present invention aims to develop a method for the efficient and selective conversion of methane into nitromethane under mild conditions, specifically relating to a method for the continuous flow photocatalytic nitration of methane to prepare nitromethane.

[0007] In a first aspect of the present invention, a method for preparing nitromethane by continuous flow photocatalytic nitration of methane is provided, the method comprising the steps of: (1) Continuous supply and mixing of raw materials: The liquid reactant is obtained by mixing nitrating agent, additive, iron-based photocatalyst and solvent; the methane gas and the liquid reactant are respectively fed into one or more series continuous flow microchannel reactors through independent conveying systems. With the help of their static mixing structure and / or microchannel dispersion structure, the methane gas is highly dispersed in the liquid reactant to form a gas-liquid two-phase reaction system. (2) Photocatalytic nitration reaction: Under light irradiation, the reaction system in the continuous flow microchannel reactor is controlled to carry out a nitration reaction to generate a reaction effluent containing nitromethane; (3) Continuous separation and recovery of products: The reaction effluent obtained in step (2) is subjected to continuous gas-liquid separation to obtain a gas phase rich in unreacted methane and a liquid phase containing nitromethane. The liquid phase is separated and purified to obtain nitromethane products. The nitrating agent is selected from the following group: aqueous nitric acid solution, fuming nitric acid, or a nitrification system composed of nitrate and protic acid; The additive is selected from chloride ion sources and / or nitrites.

[0008] In another preferred embodiment, the nitrating agent is selected from the group consisting of: aqueous nitric acid solution or fuming nitric acid.

[0009] In another preferred embodiment, the nitrifying agent is an aqueous solution of nitric acid with a concentration of 65-70 wt.%.

[0010] In another preferred embodiment, the iron-based photocatalyst is an iron salt or iron complex selected from the group consisting of: ferric chloride (FeCl3), ferric bromide (FeBr3), ferric trifluoromethanesulfonate (Fe(OTf)3), ferric trifluoroacetate (Fe(TFA)3), ferric sulfate (Fe2(SO4)3), ferric nitrate (Fe(NO3)3), ferric phosphate (FePO4), ferric tetrafluoroborate (Fe(BF4)3), ferric perchlorate (Fe(ClO4)3), ferric oxalate (Fe2(C2O4)3), ferric acetylacetone (Fe(acac)3), ferric p-toluenesulfonate (Fe(OTs)3), ferric ethylenediaminetetraacetate complexes, or combinations thereof; Alternatively, the iron-based photocatalyst is a ferrous salt selected from the group consisting of ferrous chloride (FeCl2), ferrous bromide (FeBr2), ferrous sulfate (FeSO4), ferrous acetate (Fe(CH3CO2)2), ferrous carbonate (FeCO3), or a combination thereof; The iron-based photocatalyst can be in its anhydrous form, hydrate form, or corresponding iron complex form.

[0011] In another preferred embodiment, the iron-based photocatalyst comprises a ligand selected from the group consisting of pyridines, bipyridines, terpyridines, tetrapyridines, phenanthroline ligands, Py-Box ligands, tetraphenylporphyrin ligands and their derivatives, or combinations thereof, preferably bipyridine ligands.

[0012] In another preferred embodiment, the iron-based photocatalyst may be a pre-synthesized iron complex or may be formed by the in-situ complexation of iron salt and ligand in the reaction system.

[0013] In another preferred embodiment, the solvent is selected from the group consisting of ester solvents, halogenated hydrocarbon solvents, alcohol solvents, nitrile solvents, amide solvents, sulfoxide solvents, water, and mixtures thereof.

[0014] In another preferred embodiment, the solvent is selected from the group consisting of ethyl acetate, hexafluoroisopropanol, acetonitrile, water, or combinations thereof, preferably acetonitrile.

[0015] In another preferred embodiment, the chloride ion source is selected from the group consisting of lithium chloride, ammonium chloride, sodium chloride, potassium chloride, magnesium chloride, calcium chloride, aluminum chloride, cobalt chloride, nickel chloride, copper chloride, zinc chloride, titanium chloride, vanadium chloride, manganese chloride, hydrochloric acid, pyridine hydrochloride, tetrabutylammonium chloride, or combinations thereof.

[0016] In another preferred embodiment, the chloride ion source is selected from hydrochloric acid and / or ammonium chloride.

[0017] In another preferred embodiment, the nitrite additive is selected from the group consisting of nitrite, sodium nitrite, potassium nitrite, silver nitrite, calcium nitrite, tetrabutylammonium nitrite, sodium hexanotopcobaltate, sodium cobalt nitrite, potassium cobalt nitrite, or combinations thereof.

[0018] In another preferred embodiment, the nitrite additive is selected from sodium nitrite and / or potassium nitrite.

[0019] In another preferred embodiment, the illumination conditions are: ultraviolet light and / or visible light with wavelengths of 300–780 nm as the light source.

[0020] In another preferred embodiment, the illumination conditions are: an LED with a wavelength of 360–420 nm is used as the light source.

[0021] In another preferred embodiment, the temperature of the nitration reaction in step (2) is 5°C to 80°C.

[0022] In another preferred embodiment, the nitration reaction in step (2) is carried out at room temperature of 20°C to 30°C.

[0023] In another preferred embodiment, in step (1), the molar ratio of methane gas to nitric acid (HNO3), the nitrifying agent in the liquid reactants, is 1:1 to 20:1.

[0024] In another preferred embodiment, the molar ratio of the iron salt or iron complex to the ligand is 1:0 to 1:10, preferably 1:1 to 1:3.

[0025] In another preferred embodiment, the molar ratio of the iron salt or iron complex to the chloride-containing additive is 1:0 to 1:1000, preferably 1:10 to 1:50.

[0026] In another preferred embodiment, the molar ratio of the nitrifying agent to the nitrite additive is 1:0 to 1:0.1, preferably 1:0.0001 to 1:0.001.

[0027] In another preferred embodiment, the residence time of the reactants in the flow microchannel reactor is 1 to 60 minutes, preferably 5 to 20 minutes.

[0028] In another preferred embodiment, the flow microchannel reactor is a glass microchannel reactor module with a total internal retention volume of 40-45 mL, and the reactor module is a single module or multiple modules connected in series.

[0029] In another preferred embodiment, the material of the flow microchannel reactor is selected from borosilicate glass or other equivalent materials that combine good light transmittance and corrosion resistance.

[0030] In another preferred embodiment, the progress and product analysis of the nitration reaction can be monitored and qualitatively and quantitatively analyzed using conventional methods in the art, such as gas chromatography-flame ionization detector (GC-FID) and / or gas chromatography-mass spectrometry (GC-MS).

[0031] It should be understood that, within the scope of this invention, the above-described technical features of this invention and the technical features specifically described below (such as in the embodiments) can be combined with each other to form new or preferred technical solutions. Due to space limitations, they will not be described in detail here. Attached Figure Description

[0032] Figure 1 A diagram of a continuous flow microchannel photoreactor device is shown. Detailed Implementation

[0033] Through extensive and in-depth research and numerous experimental screenings, the inventors have unexpectedly developed a method for the continuous-flow photocatalytic nitration of methane to produce nitromethane. This invention uses nitric acid as the nitrating agent. Under light irradiation, and with the aid of a reaction system consisting of an iron-based photocatalyst, additives, and a solvent, the continuous conversion of methane to nitromethane is achieved in a continuous-flow microchannel reactor. This method operates under mild conditions, can be carried out near room temperature, and requires only moderate reaction pressure, ensuring high operational safety. Furthermore, the reaction process does not use highly toxic reagents, meeting environmental protection requirements. The catalyst used is based on abundant iron, offering advantages such as low cost, good stability, and recyclability, demonstrating promising prospects for industrial application. In addition, this method exhibits high selectivity, with a C–N product (nitromethane) to C–O product (methyl nitrate) selectivity ratio exceeding 10:1, which simplifies subsequent separation and purification processes. Based on these findings, the inventors completed this invention.

[0034] Terminology Explanation The method for preparing nitromethane by continuous flow photocatalytic nitration of methane according to the present invention This invention uses nitric acid as the nitrating agent and, under light irradiation, utilizes a reaction system composed of an iron-based photocatalyst, additives, and a solvent to achieve the continuous conversion of methane into nitromethane in a continuous flow microchannel reactor.

[0035] Specifically, the method includes the following steps: (1) Continuous supply and mixing of raw materials: Methane gas and liquid reactants containing nitrating agents, additives, iron-based photocatalysts (which may contain ligands) and solvents are continuously and stably fed into one or more series continuous flow microchannel reactors through independent conveying systems; through static mixing structure and / or dispersion structure within the microchannels, the gas forms a highly dispersed gas-liquid two-phase system in the liquid phase, thereby improving mass transfer efficiency and effective contact area.

[0036] (2) Photocatalytic nitration reaction: Under ultraviolet light and / or visible light irradiation, the reaction system is controlled to carry out nitration reaction in a microchannel reactor under preset temperature and pressure conditions to generate nitromethane; the temperature and pressure are precisely controlled by the temperature control unit and the back pressure valve, and the residence time is controllable by adjusting the total flow rate.

[0037] (3) Continuous collection and purification of products: The reaction effluent from the outlet of the microchannel reactor can preferably first enter an online gas-liquid separator to separate the gas phase and liquid phase: the gas phase mainly consists of unreacted methane and a small amount of nitrogen oxides, etc., and the unreacted methane can be recovered and recycled; the liquid phase is the reaction product effluent, which is purified by distillation to obtain nitromethane product. The diagram of the continuous flow microchannel photoreactor device is shown below. Figure 1 As shown.

[0038] In the method, the methane partial pressure / reaction pressure is: the methane partial pressure is 1 to 100 bar (absolute pressure), preferably 5 to 30 bar, and more preferably 10 to 20 bar.

[0039] The nitrating agent is an aqueous solution of nitric acid, fuming nitric acid, or a nitrification system composed of nitrate and protic acid. An aqueous solution of nitric acid or fuming nitric acid is preferred, and an aqueous solution of nitric acid with a concentration of 68 wt.% is more preferred.

[0040] The iron-based photocatalyst is one or more of the following: ferric chloride (FeCl3), ferric bromide (FeBr3), ferric trifluoromethanesulfonate (Fe(OTf)3), ferric trifluoroacetate (Fe(TFA)3), ferric sulfate (Fe2(SO4)3), ferric nitrate (Fe(NO3)3), ferric phosphate (FePO4), ferric tetrafluoroborate (Fe(BF4)3), ferric perchlorate (Fe(ClO4)3), ferric oxalate (Fe2(C2O4)3), ferric acetylacetone (Fe(acac)3), ferric p-toluenesulfonate (Fe(OTs)3), ferric ethylenediaminetetraacetate complexes, and anhydrous forms, hydrates, or corresponding iron complexes of their respective ferrous salts (FeCl2, FeBr2, FeSO4, Fe(CH3CO2)2, FeCO3, etc.). Preferably, one or more of ferric chloride, ferrous chloride, ferric perchlorate, ferric nitrate, and ferric tetrafluoroborate are selected.

[0041] The ligands of the iron-based photocatalyst can be selected from one or more of pyridine, bipyridine, terpyridine, tetrapyridine, phenanthroline, Py-Box, tetraphenylporphyrin and their derivatives, with bipyridine ligands being more preferred. The iron-based photocatalyst can be a pre-synthesized iron complex or can be formed by in-situ complexation of iron salt and ligand in the reaction system.

[0042] The solvent is selected from ester solvents, halogenated hydrocarbon solvents, alcohol solvents, nitrile solvents, amide solvents, sulfoxide solvents, water, and mixtures thereof. Ethyl acetate, hexafluoroisopropanol, acetonitrile, and water are preferred, with acetonitrile being more preferred.

[0043] The additives include chloride ion sources and / or nitrites.

[0044] The chloride ion source is selected from one or more quaternary ammonium salts such as lithium chloride, ammonium chloride, sodium chloride, potassium chloride, magnesium chloride, calcium chloride, aluminum chloride, cobalt chloride, nickel chloride, copper chloride, zinc chloride, titanium chloride, vanadium chloride, manganese chloride, hydrochloric acid, pyridine hydrochloride, and tetrabutylammonium chloride; preferably hydrochloric acid and / or ammonium chloride. The nitrite additive is selected from one or more of nitrite, sodium nitrite, potassium nitrite, silver nitrite, calcium nitrite, tetrabutylammonium nitrite, sodium hexanocyanate, sodium cobalt nitrite, and potassium cobalt nitrite; preferably sodium nitrite and / or potassium nitrite.

[0045] The light source is ultraviolet light and / or visible light with a wavelength of 300–780 nm; preferably, it is an LED light source with a wavelength of 360–420 nm.

[0046] The reaction is carried out at a temperature of 5°C to 80°C, preferably at a room temperature of 20°C to 30°C.

[0047] The molar ratio of methane to nitrating agent (calculated as HNO3) is 1:1 to 20:1. The molar ratio of the iron salt or iron complex to the ligand is 1:0 to 1:10, preferably 1:1 to 1:3.

[0048] The molar ratio of the iron salt or iron complex to the chloride-containing additive is 1:0 to 1:1000, preferably 1:10 to 1:50.

[0049] The molar ratio of the nitrifying agent to the nitrite additive is 1:0 to 1:0.1, preferably 1:0.001 to 1:0.01.

[0050] The residence time of the reactants in the flow microchannel reactor is 1 to 60 minutes, preferably 5 to 20 minutes.

[0051] The flow microchannel reactor is preferably a glass microchannel reactor module with a total internal retention volume of 42 mL. The material is high borosilicate glass or equivalent corrosion-resistant and light-transmitting material, which has good light transmittance and corrosion resistance. It can be used as a single module or multiple modules connected in series.

[0052] The reaction process and product analysis can be monitored and qualitatively and quantitatively analyzed using conventional methods in the art, such as gas chromatography-flame ionization detector (GC-FID) and / or gas chromatography-mass spectrometry (GC-MS).

[0053] A method for directly converting methane into nitromethane via photocatalytic nitration in a continuous flow reactor.

[0054] Compared with the prior art, the beneficial effects of the present invention are as follows: 1. Compared with the traditional gas-phase nitration process of methane (reaction conditions of 350~450°C and 0.8~1.2 MPa), the method for preparing nitromethane described in this invention significantly reduces the reaction temperature to the room temperature range and the reaction pressure to a moderate level (0.1~0.2 MPa), thereby greatly reducing energy consumption and the requirements for the temperature and pressure resistance of equipment, while significantly improving operational safety.

[0055] 2. The preparation method of the present invention effectively improves the selectivity of the target product nitromethane (C–N bond formation) and significantly inhibits the over-oxidation of methane to C–O-containing products (such as methyl nitrate). Specifically, the selectivity ratio of C–N product (nitromethane) to C–O product (methyl nitrate) exceeds 10:1, which helps to simplify the subsequent separation and purification process.

[0056] 3. The preparation method of the present invention uses visible light as the driving energy, does not use highly toxic reagents in the reaction process, and ensures the safety and stability of the process through continuous flow process design. It has a clear industrialization implementation path and good prospects for industrial application.

[0057] 4. The catalyst used in this invention is constructed based on abundant iron elements, which is low in cost, has good stability, and has the potential for recycling, further enhancing the economic efficiency and sustainability of the technology.

[0058] The present invention will be further illustrated below with reference to specific embodiments. It should be understood that these embodiments are for illustrative purposes only and are not intended to limit the scope of the invention. Experimental methods in the following embodiments, unless otherwise specified, are generally performed under conventional conditions or as recommended by the manufacturer. Percentages and parts are by weight unless otherwise stated.

[0059] Example 1: Preparation of the reaction solution: In a 250 mL reagent bottle, add anhydrous ferric chloride (FeCl3, 4.8 mg, 0.03 mmol), 5,5'-bis(trifluoromethyl)-2,2'-bipyridine (17.5 mg, 0.06 mmol), concentrated hydrochloric acid (50 μL, 0.6 mmol), sodium nitrite (3.4 mg, 0.05 mmol), and 68 wt.% nitric acid aqueous solution (4.5 mL). Add acetonitrile to a total volume of 150 mL and mix thoroughly to obtain a homogeneous reaction solution. The concentration of nitric acid is approximately 0.5 mol / L (calculated as HNO3).

[0060] System Preparation and Reaction Operation: A glass microchannel reactor module was used (one or more modules can be connected in series; in this embodiment, multiple modules are connected in series, maintaining a volume of 42 mL). A 400 nm LED array light source and its corresponding cooling system were turned on, and the reactor jacket temperature was set to 25 °C, and the back pressure valve was set to 15 bar. First, pure acetonitrile was pumped into the system at a flow rate of 1.0 mL / min for rinsing and wetting. Then, the methane cylinder was opened, and the methane flow rate was controlled at 45 mL / min (standard conditions) using a mass flow controller, with a methane to nitric acid molar ratio of 4:1. After the system pressure stabilized, the injection pump was stopped, and the injection line was switched to the prepared reaction solution. The reaction solution was pumped into the reactor at a flow rate of 1.0 mL / min. Based on the feed, the methane to nitric acid molar ratio was approximately 4:1. A continuous flow photocatalytic nitration reaction was carried out under illumination at 25 °C and 15 bar, with a residence time of approximately 10 minutes in the reactor. After the system had been running stably for 30 minutes, product collection began at the outlet.

[0061] Product Analysis and Purification: 100 mL of the reaction effluent was continuously collected. A portion of the sample was quantitatively analyzed using gas chromatography-flame ionization detector (GC-FID) equipped with a DB-WAX column. The results showed that the major product was nitromethane (CH3NO2), with a peak area ratio (i.e., CN / CO selectivity) of approximately 11:1 with the major byproduct methyl nitrate (CH3ONO2). The collected 100 mL of reaction solution was subjected to atmospheric distillation, and the fraction collected at 98–106 °C yielded 1.31 g (21.5 mmol) of a colorless, transparent liquid product.

[0062] The calculated yield of nitromethane is 18.9 g / d, with a conversion rate of approximately 10.8% based on the feed methane, approximately 43% based on the nitric acid, a turnover number (TON) of 1074 based on iron, and a purity of greater than 98% for nitromethane.

[0063] The retention time is the total retention volume of the reactor divided by the total gas and liquid flow rates. The specific calculation process is as follows: 42mL÷[1 mL / min + (45 mL / min)÷15 bar) ]= 10.5 min.

[0064] The yield per unit time is the mass of nitromethane obtained in 100 mL of reaction solution divided by the running time to obtain 100 mL of reaction solution (100 mL ÷ 1 mL / min = 100 min). The specific calculation process is as follows: 1.31 g ÷ 100 min = 13.1 mg / min = 18.9 g / d.

[0065] The methane conversion rate is equal to the number of moles of nitromethane obtained in 100 mL of reaction solution divided by the number of moles of methane added during the reaction run (100 min). The specific calculation process is as follows: (1.31 g ÷ 61 g / mol) ÷ [(45 mL / min × 100 min ÷ 1000) ÷ 22.4 L / mol] = 10.8%.

[0066] The conversion rate of nitric acid is calculated as follows: (1.31 g ÷ 61 g / mol) ÷ [0.5 M × 0.1 L] = 43%.

[0067] The turnover rate of the iron catalyst is the number of moles of nitromethane obtained in 100 mL of reaction solution divided by the number of moles of iron catalyst added in 100 mL of reaction solution. The specific calculation process is as follows: (1.31 g ÷ 61 g / mol) ÷ [0.03 mmol × (100 mL ÷ 150 mL) ÷ 1000] = 1074.

[0068] Example 2: Based on Example 1, the flow rate of methane gas was changed from 45 mL / min (standard conditions) to 55 mL / min (standard conditions), and the residence time was changed to 9 min. The molar ratio of methane to nitric acid was 5:1 based on the feed, and other conditions remained unchanged.

[0069] GC-FID showed a CN / CO selectivity of approximately 11:1. After atmospheric distillation, 1.20 g (19.7 mmol) of a colorless, transparent liquid product was obtained. Based on this, the yield per unit time was approximately 17.3 g / d; the conversion of methane was approximately 8.0%; the conversion of nitric acid was approximately 39%; the TON of the iron catalyst was approximately 984; and the purity of nitromethane was greater than 98%.

[0070] Example 3 (without iron salts, control): Based on Example 1, ferric chloride (FeCl3) was not added, while all other conditions remained unchanged.

[0071] GC-FID showed a CN / CO selectivity of approximately 2.1:1. After atmospheric distillation, 0.1 g (1.6 mmol) of product was obtained. Based on this, the yield per unit time is approximately 1.4 g / d; the conversion of methane is approximately 0.8%; and the conversion of nitric acid is approximately 3.3%. Since no iron catalyst was added, TON is not applicable.

[0072] Example 4 (without ligand): Based on Example 1, the ligand 5,5'-bis(trifluoromethyl)-2,2'-bipyridine was not added, and other conditions remained unchanged.

[0073] GC-FID showed a CN / CO selectivity of approximately 7.3:1. After atmospheric distillation, 0.82 g (13.5 mmol) of product was obtained. Based on this, the yield per unit time is approximately 11.8 g / d; the conversion of methane is approximately 6.7%; the conversion of nitric acid is 27%; and the TON of the iron catalyst is approximately 672.

[0074] Example 5 (without concentrated hydrochloric acid additive): Based on Example 1, concentrated hydrochloric acid was not added, and all other conditions remained unchanged.

[0075] GC-FID showed a CN / CO selectivity of approximately 10.5:1. After atmospheric distillation, 1.02 g (16.7 mmol) of product was obtained. Based on this, the yield per unit time is calculated to be approximately 14.7 g / d; the conversion of methane is approximately 8.4%; the conversion of nitric acid is 33%; and the TON of the iron catalyst is approximately 836.

[0076] Example 6 (without sodium nitrite additive): Based on Example 1, sodium nitrite was not added, and all other conditions remained unchanged.

[0077] GC-FID showed a CN / CO selectivity of approximately 11:1. After atmospheric distillation, 0.92 g (15.1 mmol) of product was obtained. Based on this, the yield per unit time is calculated to be approximately 14.1 g / d; the conversion of methane is approximately 7.5%; the conversion of HNO3 to nitric acid is 30%; and the TON of the iron catalyst is approximately 754.

[0078] Example 7 (Light-protected control experiment) Based on Example 1, the 400 nm LED array light source and its supporting cooling system were not turned on, while other conditions remained unchanged. After the reaction was completed, GC-FID analysis of the reaction effluent showed no detection of the target product, nitromethane.

[0079] Example 8 (Experiment with different wavelengths of light) Based on Example 1, the light source was replaced with a 460 nm LED array (the light intensity was calibrated to be equivalent to a 400 nm light source), while other conditions remained unchanged. GC-FID analysis of the reaction effluent showed that the peak area ratio (CN / CO selectivity) of nitromethane (CH3NO2) to the byproduct methyl nitrate (CH3ONO2) was approximately 15:1. After atmospheric distillation, 0.98 g (16.1 mmol) of a colorless, transparent liquid product was obtained. The calculated yield of nitromethane was 14.1 g / d, based on a methane conversion of approximately 8.1%, a nitric acid conversion of approximately 32%, an iron turnover number (TON) of 800, and a product purity greater than 98%. The experimental data are summarized in Table 1.

[0080] Table 1: Summary List of Data Note 1: Standard conditions refer to the reaction solution components of Example 1.

[0081] The above data indicate that illumination is a prerequisite for initiating the photocatalytic nitration reaction of this invention (Example 7), and the wavelength of the light source has a significant impact on the reaction performance (Example 8). Under the preferred wavelength of 400 nm (Example 1), the highest reaction efficiency (yield 18.9 g / d, TON 1066) and excellent CN / CO selectivity (11:1) can be obtained; while when the wavelength is shifted to 460 nm (Example 8), although the selectivity is slightly improved (15:1), the overall yield and catalyst efficiency decrease significantly. This proves that this invention has a specific selectivity for light wavelength, and the vicinity of 400 nm is the key excitation wavelength for achieving efficient and highly selective conversion of methane. At the same time, the comparison between Example 1 and Examples 3-6 further confirms that the catalytic system composed of iron-based photocatalyst, ligand, chloride ion source and nitrite has a significant synergistic effect, and the absence of any one element will lead to a significant reduction in performance.

[0082] All documents mentioned in this invention are incorporated herein by reference as if each document were individually incorporated by reference. Furthermore, it should be understood that after reading the foregoing teachings of this invention, those skilled in the art can make various alterations or modifications to this invention, and these equivalent forms also fall within the scope defined by the appended claims.

Claims

1. A method for preparing nitromethane by continuous flow photocatalytic nitration of methane, characterized in that, The method includes the following steps: (1) Continuous supply and mixing of raw materials: The liquid reactant is obtained by mixing nitrating agent, additive, iron-based photocatalyst and solvent; the methane gas and the liquid reactant are respectively fed into one or more series continuous flow microchannel reactors through independent conveying systems. With the help of their static mixing structure and / or microchannel dispersion structure, the methane gas is highly dispersed in the liquid reactant to form a gas-liquid two-phase reaction system. (2) Photocatalytic nitration reaction: Under light irradiation, the reaction system in the continuous flow microchannel reactor is controlled to carry out a nitration reaction to generate a reaction effluent containing nitromethane; (3) Continuous separation and recovery of products: The reaction effluent obtained in step (2) is subjected to continuous gas-liquid separation to obtain a gas phase rich in unreacted methane and a liquid phase containing nitromethane. The liquid phase is separated and purified to obtain nitromethane products. The nitrating agent is selected from the following group: aqueous nitric acid solution, fuming nitric acid, or a nitrification system composed of nitrate and protic acid; The additive is selected from chloride ion sources and / or nitrites.

2. The method as described in claim 1, characterized in that, The nitrifying agent is selected from the group consisting of: aqueous nitric acid solution or fuming nitric acid.

3. The method as described in claim 1, characterized in that, The iron-based photocatalyst is an iron salt or iron complex selected from the group consisting of: ferric chloride (FeCl3), ferric bromide (FeBr3), ferric trifluoromethanesulfonate (Fe(OTf)3), ferric trifluoroacetate (Fe(TFA)3), ferric sulfate (Fe2(SO4)3), ferric nitrate (Fe(NO3)3), ferric phosphate (FePO4), ferric tetrafluoroborate (Fe(BF4)3), ferric perchlorate (Fe(ClO4)3), ferric oxalate (Fe2(C2O4)3), ferric acetylacetone (Fe(acac)3), ferric p-toluenesulfonate (Fe(OTs)3), ferric ethylenediaminetetraacetate complexes, or combinations thereof; Alternatively, the iron-based photocatalyst is a ferrous salt selected from the group consisting of ferrous chloride (FeCl2), ferrous bromide (FeBr2), ferrous sulfate (FeSO4), ferrous acetate (Fe(CH3CO2)2), ferrous carbonate (FeCO3), or a combination thereof; The iron-based photocatalyst can be in its anhydrous form, hydrate form, or corresponding iron complex form.

4. The method as described in claim 1, characterized in that, The iron-based photocatalyst comprises a ligand selected from the group consisting of pyridines, bipyridines, terpyridines, tetrapyridines, phenanthroline ligands, Py-Box ligands, tetraphenylporphyrin ligands and their derivatives, or combinations thereof, preferably bipyridine ligands.

5. The method as described in claim 1, characterized in that, The solvent is selected from the group consisting of ester solvents, halogenated hydrocarbon solvents, alcohol solvents, nitrile solvents, amide solvents, sulfoxide solvents, water, and mixtures thereof.

6. The method as described in claim 1, characterized in that, The illumination conditions are: ultraviolet light and / or visible light with wavelengths of 300–780 nm as the light source.

7. The method as described in claim 1, characterized in that, The temperature of the nitration reaction in step (2) is 5°C to 80°C.

8. The method as described in claim 1, characterized in that, In step (1), the molar ratio of methane gas to nitric acid (HNO3), the nitrifying agent in the liquid reactants, is 1:1 to 20:

1.

9. The method as described in claim 1, characterized in that, The residence time of the reactants in the flow microchannel reactor is 1 to 60 minutes, preferably 5 to 20 minutes.

10. The method as described in claim 1, characterized in that, The flow microchannel reactor is a glass microchannel reactor module with a total internal retention volume of 40-50 mL, and the reactor module can be a single module or multiple modules connected in series.

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