JP7696155B2 - Method for producing alkane oxide - Google Patents

Description

The present invention relates to a method for producing alkane oxides.

Among the petroleum components, alkanes are effectively used as an energy source. However, alkanes are difficult to chemically change and are difficult to use as raw materials (reaction substrates) for chemical reactions. Furthermore, among alkanes, those that are gaseous at room temperature and pressure are expensive to store and transport. Among these gaseous alkanes, methane also acts as a greenhouse gas.

Therefore, studies have been conducted to oxidize alkanes and use the resulting oxidation products as raw materials for chemical reactions.

In the following description, the oxidation products obtained by oxidizing alkanes are referred to as "alkane oxides." Alkane oxides include alcohols, aldehydes, ketones, and peroxides. The above reactions can expand the uses of alkanes.
Alkanes that are gaseous at normal temperature and pressure are called "gaseous alkanes." Examples of gaseous alkanes include methane, ethane, propane, and butane.

By oxidizing gaseous alkanes to obtain alkane oxides through the above-mentioned oxidation reaction, the range of uses will expand. Furthermore, many of the alkane oxides obtained are compounds that are liquid at room temperature and pressure, making them easy to handle.

On the other hand, alkanes, especially gaseous alkanes, have high C-H bond dissociation energy, and oxidation reactions often require high temperature conditions or conditions using a highly reactive oxidizing agent. In recent years, therefore, techniques have been reported for oxidizing alkanes at around room temperature (see, for example, Patent Document 1). In addition, techniques have been reported for oxidizing alkanes using oxygen as an oxidizing agent (see, for example, Non-Patent Document 1).

JP 2020-138929 A

L.D.Pfefferle, J. Am. Chem. Soc. 2020, 142, 17119-17130

However, the method described in Patent Document 1 uses highly reactive hydrogen peroxide as an oxidizing agent. The method described in Non-Patent Document 1 requires high reaction temperatures of more than 100°C. For example, technology for oxidizing alkanes using oxygen as an oxidizing agent in a low-temperature environment, such as room temperature, has not been put to practical use, and improvements have been required.

The present invention was made in consideration of these circumstances, and aims to provide a method for producing alkane oxides using oxygen as an oxidizing agent in a temperature environment lower than conventional methods.

To solve the above problems, one aspect of the present invention includes the following aspects:

[1] A method for producing an alkane oxide, comprising the step of irradiating light to a reaction solution containing an alkane, parabenzoquinone having an electron-withdrawing group, a metal salt, and water in an oxygen-containing atmosphere, the reaction temperature of the step being equal to or higher than 0°C and lower than the boiling point of the solvent of the reaction solution, the metal element of the metal salt being at least one selected from the group consisting of Cr, Mn, Fe, Co, Ni, and Cu, and the wavelength band of the light including the maximum absorption wavelength of the parabenzoquinone.

[2] The method for producing an alkane oxide according to [1], wherein the reaction solution further contains nitrite ions.

[3] The method for producing an alkane oxide according to [1] or [2], wherein the parabenzoquinone is 2,3-dichloro-5,6-dicyano-p-benzoquinone or tetrachloro-1,4-benzoquinone.

[4] The method for producing an alkane oxide according to any one of [1] to [3], wherein the metal element is Cu.

[5] The method for producing an alkane oxide according to [4], wherein the metal salt is copper acetate.

[6] The method for producing an alkane oxide according to any one of [1] to [5], wherein the solvent of the reaction solution is acetonitrile.

[7] The method for producing an alkane oxide according to any one of [1] to [6], wherein the reaction temperature is 10°C or higher and 40°C or lower.

[8] The method for producing an alkane oxide according to any one of [1] to [7], wherein the alkane includes at least one selected from the group consisting of methane, ethane, propane, and butane.

The present invention provides a method for producing alkane oxides using oxygen as an oxidizing agent in a lower temperature environment than conventional methods.

FIG. 1 is a diagram showing the oxidation reaction of this embodiment. FIG. 2 is a scatter plot of the results of Experimental Example 2, with the horizontal axis representing the metal salt concentration (unit: mmol/L) and the vertical axis representing the concentration of total alkane oxides (unit: mmol/L). FIG. 3 is a scatter plot of the results of Experimental Example 3, with the DDQ concentration (unit: mmol/L) on the horizontal axis and the concentration of total alkane oxides (unit: mmol/L) on the vertical axis. FIG. 4 is a scatter plot of the results of Experimental Example 4, with the sodium nitrite concentration on the horizontal axis (unit: mmol/L) and the concentration of total alkane oxides on the vertical axis (unit: mmol/L). FIG. 5 is a scatter plot of the results of Experimental Example 5, with the horizontal axis representing the water concentration (unit: mmol/L) and the vertical axis representing the concentration of total alkane oxides (unit: mmol/L). FIG. 6 is a scatter plot of the results of Experimental Example 6, with the intensity of irradiated light on the horizontal axis (unit: %) and the concentration of total alkane oxides on the vertical axis (unit: mmol/L). FIG. 7 is a scatter plot of the results of Experimental Example 7, with the propane pressure (unit: MPa) on the horizontal axis and the concentration of total alkane oxides (unit: mmol/L) on the vertical axis. FIG. 8 is a scatter plot of the results of Experimental Example 8, with the horizontal axis representing reaction time (unit: hours) and the vertical axis representing the concentration of total alkane oxides (unit: mmol/L).

The method for producing an alkane oxide according to this embodiment includes a step of irradiating a reaction solution containing an alkane, parabenzoquinone having an electron-withdrawing group, a metal salt, and water with light in an oxygen-containing atmosphere. In the method for producing an alkane oxide according to this embodiment, the oxygen molecules contained in the atmosphere function as an oxidizing agent.

Here, the "oxidizing agent" in this embodiment refers to a compound that has the function of being reduced by receiving electrons taken from a reaction substrate in an oxidation reaction and transferring them to the outside of the reaction system. In this sense, oxygen molecules are reduced by receiving electrons taken from a reaction substrate in an oxidation reaction and transfer the electrons to the outside of the reaction system in the form of water molecules. Details will be described later.
The following explains each in order.

"Oxygen-containing atmosphere" refers to an environment that contains oxygen. Typically, this is air.

The reaction temperature of the step is 0° C. or higher and lower than the boiling point of the solvent of the reaction solution. The reaction temperature is preferably around room temperature, since it is easier to control the reaction conditions. Here, "room temperature" refers to 25° C. The reaction temperature is preferably 5° C. or higher, and more preferably 10° C. or higher. The reaction temperature is preferably 50° C. or lower, and more preferably 40° C. or lower.
The upper limit and the lower limit of the reaction temperature can be combined in any desired manner.

In the present invention, the alkane, which is the reaction substrate, may be liquid or gaseous under atmospheric pressure and room temperature. In the method for producing an alkane oxide of the present invention, gaseous alkanes can also be suitably oxidized.

The alkane that is the reaction substrate may include a gaseous alkane. That is, the alkane may include at least one selected from the group consisting of methane, ethane, propane, and butane.

Parabenzoquinones that have an electron-withdrawing group function as oxidation catalysts in oxidation reactions. Examples of electron-withdrawing groups include halogen atoms and cyano groups. As such parabenzoquinones, 2,3-dichloro-5,6-dicyano-p-benzoquinone (DDQ) or tetrachloro-1,4-benzoquinone (chloranil) are preferred.

However, it has been confirmed that if parabenzoquinone (or hydroquinone), which does not have an electron-withdrawing group, is used, the hydroquinone decomposes during the reaction.

The metal element contained in the metal salt is a first transition element, and is at least one selected from the group consisting of Cr, Mn, Fe, Co, Ni, and Cu. The metal element is preferably Cu. These metal elements have redox activity, and are therefore believed to promote the desired oxidation reaction described below.

On the other hand, if the metal element contained in the metal salt is, for example, zinc (Zn), which is redox inactive, the desired oxidation reaction is thought to proceed.

The anion of the metal salt is not particularly limited as long as it constitutes a metal salt soluble in a solvent described below. As the type of anion, nitrate ion (NO ₃ ⁻ ), acetate ion, perchlorate ion (ClO ₄ ⁻ ), sulfate ion (SO ₄ ²⁻ ) are preferred, and acetate ion is more preferred, because the metal salts that constitute the anion are easily available. Chloride ion can also be used as the anion.

A preferred metal salt is, for example, copper acetate (Cu(OAc) ₂ ).

The metal salt used may have water of hydration. For example, the copper acetate may be Cu(OAc) ₂ .H ₂ O containing water of hydration.

The solvent is a polar solvent that can dissolve alkanes, parabenzoquinone, metal salts, and water, and is relatively less susceptible to oxidation than the alkane, which is the reaction substrate in the oxidation reaction. Acetonitrile is a suitable example of such a solvent.

When a gaseous alkane is used as the alkane, the gaseous alkane exists in the gas phase of the reaction environment together with oxygen. When acetonitrile is used as the solvent, the alkane in the gas phase can be suitably dissolved and used in the oxidation reaction.

In addition, when the oxidation reaction of this embodiment is carried out under a high pressure environment, water can be used as the solvent. Water is less susceptible to oxidation reactions than alkanes, so the problem of the solvent being oxidized and decomposed during the reaction can be suppressed.

Also, since the solubility of alkanes in water is low, it is difficult to dissolve sufficient alkanes in the reaction solution at normal pressure, making it difficult for the oxidation reaction to proceed. Therefore, when using water as a solvent, it is advisable to carry out the reaction in a high-pressure environment to promote the dissolution of alkanes from the gas phase into the solvent and to proceed with the oxidation reaction.

The wavelength band of the light irradiated to the reaction solution includes the maximum absorption wavelength of the parabenzoquinone used. By irradiating the reaction solution with light of such wavelength, the parabenzoquinone absorbs the irradiated light and becomes excited, which causes the oxidation reaction of the alkane described below to proceed.

The light to be irradiated can be, for example, visible light with the ultraviolet region filtered out. Such light can be adjusted by emitting light from a light source that emits light including visible light, and passing the emitted light through a UV filter that filters out ultraviolet light of, for example, 390 nm or less.

The reaction solution preferably further contains nitrite ions (NO ₂ ⁻ ). Nitrite ions are believed to contribute to the catalytic cycle in the oxidation reaction. Therefore, the presence of nitrite ions in the reaction solution can promote the oxidation reaction of alkanes.

The concentration of nitrite ions can be controlled by adding a nitrite salt such as _NaNO2 to the reaction solution.

Fig. 1 is a diagram showing the oxidation reaction of this embodiment. In Fig. 1, DDQ is used as parabenzoquinone. In Fig. 1, the ligand represented by L can be, for example, an anion such as acetate ion or nitrite ion present in the reaction system, or a water molecule that hydrates a metal ion.
The inventors consider the alkane oxidation reaction in this embodiment as follows.

First, when the reaction solution is irradiated with light, DDQ becomes a chemical species (oxidizing active species) in a triplet excited state ( ³ DDQ ^* ) (symbol (I) in the figure). The resulting chemical species has a high reduction potential (E _red = 3.18 V vs SCE). Therefore, it is considered that the resulting chemical species oxidizes the metal salt using the oxygen atoms that constitute the water molecules (symbol (II) in the figure). More specifically, in the reaction system, the metal salt is coordinated with water molecules to form an aqua complex (LM ⁿ⁺ -OH ₂ ). The resulting aqua complex is oxidized by electron transfer from ³ DDQ ^* .

The chemical species (oxidizing active species: LM( ^{n +2)+} =O, or LM ⁽ⁿ⁺¹⁾⁺ -O ^. ) generated by the oxidation of the metal salt-aqua complex (LMn+ _-OH2 ) reacts with the alkane in the system and oxidizes it (indicated as (III) in the diagram). At the same time, the chemical species donates an oxygen atom to the alkane and reacts with the water molecules in the system to regenerate the metal salt-aqua complex.

On the other hand, DDQ in the triplet excited state oxidizes the metal salt-aqua complex to become a reduced form (DDQH ₂ ) (indicated as (IV) in the diagram). The reduced form is oxidized by oxygen molecules (O ₂ ) in the system, and DDQ is regenerated. The oxygen molecules are reduced to water molecules, which are then transported outside the reaction system. The oxygen molecules function as an oxidizing agent in the oxidation reaction shown in FIG. 1.

At this time, if nitrite ions are present in the system, the nitrogen dioxide produced by the reduction of the nitrite ions oxidizes _DDQH2 , promoting the reaction of regenerating DDQ. Nitric oxide produced by the oxidation of _DDQH2 is oxidized by oxygen in the system, regenerating nitrogen dioxide (indicated by symbol (V) in the diagram). The oxygen molecules are reduced to water molecules and move out of the reaction system.

In other words, the oxidation reaction of alkanes in this embodiment is believed to be contributed by cycle A of oxidation of alkanes by chemical species produced by oxidation of metal salt-aqua complexes, and cycle B of regeneration of DDQ, which oxidizes the metal salt-aqua complexes.

It is believed that other chemical species in the above reaction cycle also contribute to the reaction. A peroxy radical (ROO.) is generated when a radical (R.) generated from an alkane (R-H) combines with oxygen. It is believed that the generated peroxy radical oxidizes the reduced form of DDQ, promoting DDQ regeneration cycle B. When the reaction products are analyzed, alkane peroxides (ROOH) are detected as alkane oxides, which suggests that the above reaction is also occurring.

The controlling factors of the above catalytic reaction can be explained as follows:

(Light Intensity)
As described above, the catalytic reaction occurring in the present invention is triggered by photoexcitation of DDQ, and therefore, as the amount of DDQ in the triplet excited state increases with an increase in the amount of light irradiated, the oxidation reaction is accelerated.

(Oxygen Concentration)
In the above catalytic reaction, it is believed that oxygen molecules contribute to the oxidation reaction of the reduced form of DDQ, _DDQH2 , and regenerate DDQ. Therefore, it is believed that an increase in the amount of oxygen in the system makes it easier for the DDQ regeneration cycle B to run, promoting the oxidation reaction.

The oxygen concentration in the system can be controlled by controlling the partial pressure of oxygen in the gas phase.

(water concentration)
In the above catalytic reaction, alkanes are oxidized by oxygen atoms that compose the water molecules in the system. Therefore, it is believed that as the molecular weight of water increases, the alkane oxidation cycle A becomes easier to operate. On the other hand, if the amount of water in the system increases too much, each chemical species may react with water, deactivating the reaction. Therefore, it is believed that there is an optimal value for the water concentration in the system.

The optimum water concentration in the system is thought to vary depending on the reaction conditions. Factors that affect the reaction conditions include the type of alkane, the type of metal salt, the concentration of the metal salt, the type of parabenzoquinone, the concentration of parabenzoquinone, the amount of light irradiation, the oxygen concentration, and the concentration of nitrite ions. Therefore, it is advisable to determine the optimum water concentration in the system in advance through preliminary experiments.

(Nitrite ion concentration)
In the above catalytic reaction, it is believed that the nitrite ions in the system contribute to the oxidation reaction of the reduced form of DDQ, _DDQH2 , and regenerate DDQ. Therefore, when the amount of nitrite ions in the system increases, the DDQ regeneration cycle B becomes easier to operate. On the other hand, when the amount of nitrite ions in the system increases too much, the nitrite ions tend to cause a side reaction of reducing DDQ before it is photoexcited (symbol (VI) in the figure), and there is a risk that the DDQ contributing to the oxidation cycle A of the alkane will decrease. Therefore, it is believed that there is an optimum value for the concentration of nitrite ions in the system.

The optimum concentration of nitrite ions in the system is thought to vary depending on the reaction conditions. Factors that affect the reaction conditions include the type of alkane, the type of metal salt, the concentration of the metal salt, the type of parabenzoquinone, the concentration of parabenzoquinone, the amount of light irradiation, the oxygen concentration, and the water concentration. Therefore, it is advisable to determine the optimum concentration of nitrite ions in the system in advance through preliminary experiments.

(Metal Salt Concentration)
In the above catalytic reaction, the metal salt-aqua complex in the system is oxidized, and the resulting chemical species oxidizes the alkane. Therefore, as the amount of metal salt-aqua complex increases, the alkane oxidation cycle A becomes easier to operate. On the other hand, if the amount of metal salt-aqua complex in the system increases too much, the metal salt becomes too much relative to DDQ, and it is thought that the number of metal ion species whose reaction stops at "LM ^{(n+1) +} -OH" in the alkane oxidation cycle A increases. As a result, it becomes difficult for the alkane oxidation cycle A to operate, which is thought to deactivate the reaction. Therefore, it is thought that there is an optimal value for the concentration of the metal salt-aqua complex in the system.

The optimum concentration of the metal salt-aqua complex in the system is thought to vary depending on the reaction conditions. Factors that affect the reaction conditions include the type of alkane, the type of parabenzoquinone, the concentration of parabenzoquinone, the amount of light irradiation, the oxygen concentration, the water concentration, and the concentration of nitrite ions. Therefore, it is advisable to determine the optimum concentration of the metal salt in the system in advance through preliminary experiments.

(Parabenzoquinone concentration)
In the above catalytic reaction, the triplet excited chemical species generated by excitation of parabenzoquinone in the system oxidizes the metal salt-aqua complex, thereby driving the alkane oxidation cycle A. Therefore, as the amount of parabenzoquinone increases, the alkane oxidation cycle A becomes easier to operate. On the other hand, since parabenzoquinone in the system is not a compound that directly oxidizes alkanes, there is a saturation amount that contributes to the reaction. Therefore, it is thought that there is an optimum concentration of parabenzoquinone in the system.

The optimum concentration of parabenzoquinone in the system is thought to vary depending on the reaction conditions. Factors that affect the reaction conditions include the type of alkane, the type of metal salt, the concentration of the metal salt, the amount of light irradiation, the oxygen concentration, the water concentration, and the concentration of nitrite ions. Therefore, it is advisable to determine the optimum concentration of the metal salt in the system in advance through preliminary experiments.

The concentration (molar concentration) of parabenzoquinone is preferably 0.5 times or more, more preferably 1 time or more, and even more preferably 1.5 times or more, relative to the concentration (molar concentration) of the metal salt. The concentration (molar concentration) of parabenzoquinone is preferably 10 times or less, more preferably 5 times or less, and even more preferably 3 times or less, relative to the concentration (molar concentration) of the metal salt. The upper and lower limit values of the concentration ratio can be combined in any way.

Ideally, the molar concentration of parabenzoquinone should be twice that of the metal salt.

Under the reaction conditions set as described above, the method for producing alkane oxide of this embodiment can be carried out by adding an alkane to the reaction system and irradiating it with light.

It is preferable to use a reaction vessel that is transparent to the irradiated light. The reaction vessel may be a vessel in which the entire vessel is transparent to light, or a vessel having a light window in part.

The reaction vessel itself may be light-blocking, and a light source that irradiates light of the above-mentioned wavelengths may be disposed inside the reaction vessel, allowing light to be irradiated into the internal space of the vessel.

The reaction vessel may be a high-pressure reaction vessel. Using such a reaction vessel, it is possible to charge air or gaseous alkanes at atmospheric pressure or higher into the gas phase, thereby accelerating the reaction.

The method for producing alkane oxides configured as described above can provide a method for producing alkane oxides using oxygen as an oxidizing agent in a temperature environment that is lower than conventional methods.

The above describes preferred embodiments of the present invention with reference to the attached drawings, but the present invention is not limited to these examples. The shapes and combinations of the components shown in the above examples are merely examples, and various modifications can be made based on the design, specifications, etc., without departing from the spirit of the present invention.

The present invention will be described below with reference to examples, but the present invention is not limited to these examples.

In the present example, the oxidation products were identified and quantified by measuring ¹ H-NMR using a nuclear magnetic resonance apparatus (Bruker, AVANCE III, 400 MHz).

(Experimental Example 1) Types of metal salts [Experimental Example 1-1]
A metal salt (0.2 mmol/L), DDQ (1.0 mmol/L), sodium nitrite (1.0 mmol/L), and H ₂ O (1 mol/L) were dissolved in CD ₃ CN under atmospheric conditions, and the resulting reaction vessel was placed in a light-transmitting pressure-resistant reaction vessel. Cu(OAc) ₂ ·H ₂ O was used as the metal salt.

Furthermore, gaseous propane was charged until the pressure gauge attached to the reaction vessel indicated 0.7 MPa (gauge pressure). The gas phase in the reaction vessel was air.

While stirring the reaction solution, the reaction solution was irradiated with light (70% output) emitted from a white LED light source (Luminar Ace LA-HDF8010, manufactured by Hayashi Lepic Co., Ltd.) through an ultraviolet filter that cuts off light of 390 nm or less. The output of the light before passing through the filter was 24.43 W, and the output of the light after passing through the filter was 22.42 W. Light irradiation was continued for 1 hour at room temperature (25°C), and the oxidation reaction of the alkane (propane) was carried out.

After the irradiation, the reaction solution was subjected to ¹ H-NMR measurement to identify and quantify the oxidation products involved in the reaction. The quantification was carried out by the internal standard method using benzonitrile as the internal standard.

In addition, the catalyst turnover number (TON) was calculated by the following formula.
Catalytic turnover number (TON) = alkane oxide (mol)/metal salt (mol)

[Experimental Examples 1-2] to [Experimental Examples 1-7]
An oxidation reaction of an alkane was carried out in the same manner as in Experimental Example 1-1, except that the metal salt used was changed as shown in Table 1 below.

The types of oxidation products and the TON results are shown in Table 1. Experimental Examples 1-1 to 1-7 are working examples.

In Table 1, (1) to (6) indicate the types of products. The oxidation products are as follows:
(1) 2-propanol (2) acetone (3) 2-propyl hydroperoxide (4) 1-propanol (5) propanal (propionic acid)
(6) 1-propyl hydroperoxide

As a result of the evaluation, the oxidation reaction of alkanes proceeded when exposed to light at room temperature for all metal salts. In particular, it was found that Experimental Example 1-1, which used copper acetate, had a high catalyst turnover number and high reaction efficiency.

(Experimental Example 2) Metal Salt Concentration [Experimental Example 2-1] to [Experimental Example 2-12]
An oxidation reaction of an alkane was carried out in the same manner as in Experimental Example 1-1, except that copper acetate was used as the metal salt and the concentration of the metal salt was changed as shown in Table 2 below.

The types of oxidation products and the TON results are shown in Table 2. Figure 2 is a scatter plot of the results of Experimental Example 2, with the metal salt concentration on the horizontal axis (unit: mmol/L) and the concentration of total alkane oxides on the vertical axis (unit: mmol/L).

Experimental Examples 2-2 to 2-12 are working examples. Experimental Example 2-1, which does not use metal salts, is a comparative example.

In Table 2, the types of products indicated by (1) to (6) are the same as those in Table 1. The numbers in parentheses in the table indicate the concentration of each product (unit: mmol/L).

As a result of the evaluation, the amount (concentration) of alkane oxides produced was the greatest in Experimental Example 2-5, and the amount (concentration) of alkane oxides produced decreased whether the copper acetate concentration was increased or decreased. It was found that there is an optimal value for the metal salt concentration.

Experiments such as those described above can be performed as preliminary experiments to determine the concentration of metal salts.

(Experimental Example 3) Concentration of parabenzoquinone [Experimental Example 3-1] to [Experimental Example 3-9]
An oxidation reaction of an alkane was carried out in the same manner as in Experimental Example 1-1, except that the concentration of copper nitrate was 0.5 mmol/L, and DDQ was used as parabenzoquinone and the concentration of DDQ was changed as shown in Table 3 below.

The types of oxidation products and the TON results are shown in Table 3. Figure 3 is a scatter plot of the results of Experimental Example 3, with the DDQ concentration on the horizontal axis (unit: mmol/L) and the concentration of total alkane oxides on the vertical axis (unit: mmol/L).

Experimental Examples 3-2 to 3-9 are working examples. Experimental Example 3-1, which does not use DDQ, is a comparative example.

In Table 3, the types of products indicated by (1) to (6) are the same as those in Table 1. The numbers in parentheses in the table indicate the concentration of each product (unit: mmol/L).

As a result of the evaluation, the amount (concentration) of alkane oxides produced tended to increase as the DDQ concentration was increased in Experimental Examples 3-1 to 3-5, but from Example 3-6 onwards, there was no significant change in the amount (concentration) of alkane oxides produced even when the DDQ concentration was increased. It was found that there is an optimal value for the DDQ concentration.

Experiments such as those described above can be performed as preliminary experiments to determine the concentration of DDQ.

(Experimental Example 4) Nitrite Concentration [Experimental Example 4-1] to [Experimental Example 4-8]
An oxidation reaction of an alkane was carried out in the same manner as in Experimental Example 1-1, except that the concentration of copper nitrate was set to 0.5 mmol/L, sodium nitrite was used as the nitrite, and the concentration of sodium nitrite was changed. Experimental Examples 4-1 to 4-8 are all examples.

Figure 4 is a scatter plot of the results of Experimental Example 4, with the sodium nitrite concentration on the horizontal axis (unit: mmol/L) and the total alkane oxide concentration on the vertical axis (unit: mmol/L).

As a result of the evaluation, the amount (concentration) of alkane oxides produced was maximum at a sodium nitrite concentration of 2.0 mmol/L, and the amount (concentration) of alkane oxides produced decreased whether the sodium nitrite concentration was increased or decreased. It was found that there is an optimal value for the sodium nitrite concentration.

An experiment like the one above can be performed as a preliminary experiment to determine the concentration of sodium nitrite.

(Experimental Example 5) Water Concentration An oxidation reaction of an alkane was carried out in the same manner as in Experimental Example 1-1, except that the copper nitrate concentration was set to 0.5 mmol/L and the water concentration was changed. Experimental Examples 5-1 to 5-7 are all examples.

Figure 5 is a scatter plot of the results of Experimental Example 5, with the water concentration on the horizontal axis (unit: mmol/L) and the total alkane oxide concentration on the vertical axis (unit: mmol/L).

As a result of the evaluation, the amount (concentration) of alkane oxides produced was maximum at a water concentration of 0.5 mol/L, and the amount (concentration) of alkane oxides produced decreased whether the water concentration was increased or decreased. It was found that there is an optimal value for the water concentration.

An experiment like the one above can be performed as a preliminary experiment to determine the water concentration.

(Experimental Example 6) Light Intensity An oxidation reaction of an alkane was carried out in the same manner as in Experimental Example 1-1, except that the concentration of copper nitrate was set to 0.5 mmol/L and the light intensity was changed.

Figure 6 is a scatter plot of the results of Experimental Example 6, with the horizontal axis representing the intensity of the irradiated light (unit: %) and the vertical axis representing the concentration of total alkane oxides (unit: mmol/L).

The evaluation results showed that there was a linear relationship between light intensity and the total amount of oxidation products, with increasing light intensity resulting in an increase in the amount of oxidation products.

(Experimental Example 7) Alkane Concentration An oxidation reaction of an alkane was carried out in the same manner as in Experimental Example 1-1, except that the concentration of copper nitrate was set to 0.5 mmol/L and the gauge pressure of propane injected into the reaction vessel was changed.

Figure 7 is a scatter plot of the results of Experimental Example 7, with the propane pressure on the horizontal axis (unit: MPa) and the concentration of total alkane oxides on the vertical axis (unit: mmol/L).

As a result of the evaluation, it was found that there is a linear relationship between the propane pressure and the total amount of oxidation products, and that increasing the propane pressure increases the amount of oxidation products. Increasing the propane pressure in the reaction vessel also increases the amount (concentration) of propane dissolved in the reaction solution, and therefore it was found that there is a relationship in which an increase in the alkane concentration in the reaction system increases the amount of oxidation products.

(Experimental Example 8) Reaction Time An oxidation reaction of an alkane was carried out in the same manner as in Experimental Example 1-1, except that the concentration of copper nitrate was set to 0.5 mmol/L and the reaction time (light irradiation time) was changed.

Figure 8 is a scatter plot of the results of Experimental Example 8, with reaction time on the horizontal axis (unit: hours) and total alkane oxide concentration on the vertical axis (unit: mmol/L).

The evaluation revealed that the amount of oxidation products increased as the reaction time increased.

(Experimental Example 9) Reaction of methane and ethane [Experimental Example 9-1]
Under atmospheric conditions, Cu(OAc) _2.H2O ( _0.5 mmol/L), DDQ (1.0 mmol/L), sodium nitrite (2.0 mmol/L), and _H2O (0.5 mol/L) were dissolved in _CD3CN , and the resulting reaction vessel was placed in a light-transmitting pressure-resistant reaction vessel.

Furthermore, gaseous ethane was charged until the pressure gauge attached to the reaction vessel reached 0.95 MPa (gauge pressure). The gas phase in the reaction vessel was air.

While stirring the reaction solution, the reaction solution was irradiated with light (70% output) emitted from a white LED light source (Luminar Ace LA-HDF8010, manufactured by Hayashi Lepic Co., Ltd.) through an ultraviolet filter that cuts off light of 390 nm or less. The light output before passing through the filter was 24.43 W, and the light output after passing through the filter was 22.42 W. Light irradiation was continued for 15 hours at room temperature (25°C) to carry out the oxidation reaction of alkane (ethane).

After the irradiation, the reaction solution was subjected to ¹ H-NMR measurement to identify and quantify the oxidation products involved in the reaction. The quantification was carried out by the internal standard method using benzonitrile as the internal standard.

[Experimental Example 9-2]
An oxidation reaction of an alkane was carried out in the same manner as in Experimental Example 9-1, except that the concentration of Cu(OAc) _{2.H 2} O was 0.05 mmol/L.

[Experimental Example 9-3]
An oxidation reaction of an alkane was carried out in the same manner as in Experimental Example 9-1, except that methane was used as the alkane.

[Experimental Example 9-4]
An oxidation reaction of an alkane was carried out in the same manner as in Experimental Example 9-2, except that methane was used as the alkane.

The types of oxidation products and the TON results are shown in Tables 4 and 5. The numbers in parentheses in the tables indicate the concentration of each product (unit: mmol/L).

As a result of the evaluation, it was confirmed that the method for producing alkane oxides of the present invention can oxidize even gaseous alkanes with high C-H bond dissociation energy, such as methane and ethane, to obtain oxidation products.

Based on the results of Experimental Example 4, it is believed that alkane oxides can be obtained by the method for producing alkane oxides of the present invention even if an alkane with a lower C-H bond dissociation energy than methane, such as butane, a gaseous alkane, or hexane, a liquid alkane, is used as the reaction substrate.

From the above, it has been found that the present invention is useful.

Claims (7)

The method includes a step of irradiating a reaction solution containing an alkane, a parabenzoquinone having an electron-withdrawing group, a metal salt, and water with light in an oxygen-containing atmosphere,
The reaction temperature of the step is equal to or higher than 0° C. and lower than the boiling point of the solvent of the reaction solution;
The metal element contained in the metal salt is at least one selected from the group consisting of Cr, Mn, Fe, Co, Ni, and Cu,
the wavelength band of the light includes the maximum absorption wavelength of the parabenzoquinone,
The method for producing an alkane oxide , wherein the parabenzoquinone is 2,3-dichloro-5,6-dicyano-p-benzoquinone or tetrachloro-1,4-benzoquinone . The method for producing alkane oxides according to claim 1, wherein the reaction solution further contains nitrite ions. The method for producing an alkane oxide according to claim 1 or 2 , wherein the metal element is Cu. The method for producing an alkane oxide according to claim 3 , wherein the metal salt is copper acetate. The method for producing an alkane oxide according to any one of claims 1 to 4 , wherein the solvent of the reaction solution is acetonitrile. The method for producing an alkane oxide according to claim 1 , wherein the reaction temperature is 10° C. or higher and 40° C. or lower. The method for producing an alkane oxide according to any one of claims 1 to 6 , wherein the alkane includes at least one selected from the group consisting of methane, ethane, propane, and butane.

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