JP7709173B2 - How ammonia is produced - Google Patents

Description

The present invention relates to a method for producing ammonia.

In a method for producing ammonia from nitrogen molecules, there is a report on the production of ammonia using a molybdenum complex as a catalyst and water as a proton source (Non-Patent Document 1). Furthermore, there is a report on the production of ammonia using a molybdenum complex as a catalyst, samarium iodide (II) as a reducing agent, and alcohols or water as a proton source (Non-Patent Document 2).

Abstracts of the 99th Spring Annual Meeting of the Chemical Society of Japan, 2019, Lecture No. 4D1-37 Nature 2019, vol. 568 (7753), pp. 536-540

In a method for producing ammonia from nitrogen molecules, when a molybdenum complex is used as a catalyst, one important required performance from the perspective of practical application is that ammonia can be obtained immediately when needed. Therefore, in order to achieve this performance, a catalyst that can react at high speed from the early stages of the reaction is desired.

The present invention has been made to solve the above-mentioned problems, and its main objective is to create a molybdenum complex that can react rapidly from the early stages of the reaction.

In order to achieve the above-mentioned object, the inventors have attempted to improve the catalytic performance by using a wide variety of molybdenum complexes synthesized based on molecular design, and have discovered a molybdenum complex whose catalyst turnover frequency (hereinafter abbreviated as TOF) [1/min], which is the amount of material conversion per catalyst molecule per unit time, exceeds 150 at the beginning of the reaction, thereby completing the present invention.

A catalyst with a high TOF generally means that the catalyst has a high turnover rate even in a short reaction time, which is highly beneficial as it is expected to improve the efficiency of ammonia production.

That is, as a first aspect, the method for producing ammonia of the present invention is a method for producing ammonia from nitrogen molecules in the presence of a molybdenum complex, a reducing agent, and a proton source, comprising the steps of:
The molybdenum complex has the formula (1):

In formula (1), R ¹ and R ² each independently represent an alkyl group having 3 to 6 carbon atoms.
X is an iodine atom, a bromine atom, or a chlorine atom;
^R3 and ^R4 each represent a chlorine atom.
is a molybdenum complex represented by the formula:
the reducing agent is a halide of a lanthanide metal (II);
the proton source is an alcohol or water;
The catalytic turnover frequency of the molybdenum complex is greater than 150;
A method for producing ammonia.
As a second aspect, the method for producing ammonia according to the first aspect, wherein a catalyst turnover frequency exceeds 250.
As a third aspect, the method for producing ammonia according to the first aspect or the second aspect, wherein the catalyst turnover frequency can be obtained within a production time of 30 minutes.

According to the ammonia production method of the present invention, by using a molybdenum complex that can react at high speed, a method is provided that can efficiently produce ammonia from the early stage of the reaction.

In this specification, "n" stands for normal, "s" stands for secondary, and "t" stands for tertiary.

"Me" represents a methyl group, "Et" represents an ethyl group, " ^tBu " represents a tertiary butyl group, "TMS" represents a trimethylsilyl group, and "thf" represents tetrahydrofuran.

Preferred embodiments of the method for producing ammonia of the present invention and the molybdenum complex represented by formula (1) used in the method for producing ammonia will be described below.
Preferred embodiments of the method and apparatus for producing ammonia according to the present invention are described below.

In the molybdenum complex represented by formula (1),
^R1 and ^R2 each independently represent an alkyl group having 3 to 6 carbon atoms. Specific examples of the alkyl group having 3 to 6 carbon atoms include an n-propyl group, an isopropyl group, an n-butyl group, an isobutyl group, an s-butyl group, a t-butyl group, an n-pentyl group, an isopentyl group, a neopentyl group, a t-pentyl group, a 1,1-dimethylpropyl group, an n-hexyl group, an isohexyl group, and a cyclohexyl group, and a t-butyl group is preferable.
X represents an iodine atom, a bromine atom, or a chlorine atom, preferably an iodine atom or a chlorine atom, and more preferably a chlorine atom.
^R3 and ^R4 each represent a chlorine atom.

In the ammonia production method of this embodiment, exposure to reducing conditions includes, for example, coexistence of a reducing agent or contact with an electrode to supply electrons.

In the method for producing ammonia according to the present embodiment, the reducing agent may be a halide (II) of a lanthanoid metal. Examples of the lanthanoid metal include La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, and Lu, and among these, Sm is preferred. Examples of the halogen include chlorine, bromine, and iodine, and among these, iodine is preferred.
As the lanthanoid metal halide (II), samarium (II) halide is preferred, and samarium (II) iodide is more preferred.

In the method for producing ammonia according to the present embodiment, examples of the proton source include alcohol and water. As the alcohol to be used, glycol may be used, or ROH (R is a linear, cyclic or branched alkyl group having 1 to 6 carbon atoms, the hydrogen atom of which may be substituted with a fluorine atom, or a phenyl group which may have an alkyl group) may be used.
Examples of glycols include ethylene glycol, propylene glycol, and diethylene glycol.
Examples of ROH include linear or branched alkyl alcohols such as methanol, ethanol, propanol, isopropanol, n-butyl alcohol, s-butyl alcohol, isobutyl alcohol, and t-butyl alcohol, cyclic alkyl alcohols such as cyclopropanol, cyclopentanol, and cyclohexanol, fluorine atom-containing alcohols such as trifluoroethyl alcohol and tetrafluoroethyl alcohol, and phenol derivatives such as phenol, cresol, and xylenol. In the method for producing ammonia according to the present embodiment, preferred proton sources are water and ethylene glycol, and water is more preferred.

In the method for producing ammonia according to the present embodiment, the production of ammonia from nitrogen molecules may be carried out in a solvent. Examples of the solvent include, but are not limited to, cyclic ether solvents, chain ether solvents, nitrile solvents, hydrocarbon solvents, and halogen-containing hydrocarbon solvents. Examples of the cyclic ether solvents include tetrahydrofuran and 1,4-dioxane. Examples of the chain ether solvents include diethyl ether, diisopropyl ether, 1,2-dimethoxyethane, and cyclopentyl methyl ether. Examples of the nitrile solvents include acetonitrile and propionitrile. Examples of the hydrocarbon solvents include aromatic hydrocarbons such as toluene and o-xylene, and saturated hydrocarbons such as hexane, heptane, and petroleum ether. Examples of the halogen-containing hydrocarbon solvents include dichloromethane, chloroform, carbon tetrachloride, 1,2-dichloroethane, 1,1,1-trichloroethane, trichloroethylene, and tetrachloroethylene. In the method for producing ammonia according to the present embodiment, the preferred solvent is tetrahydrofuran. In the method for producing ammonia according to the present embodiment, when the molybdenum complex used as the catalyst is added, a preferred solvent is dichloromethane.

The yield of ammonia produced can be measured by known methods. The amount of ammonia in the aqueous sulfuric acid solution can be determined, for example, by the known indophenol method (Analytical Chemistry, 1967, Vol. 39, pp. 971-974).

In the present invention, the performance of a molybdenum catalyst is expressed by the catalyst turnover frequency. Here, the catalyst turnover frequency (also called TOF) is the amount of material conversion that one catalyst molecule performs per unit time.

The TOF value obtained in the present invention is greater than 150, more preferably greater than 200, and even more preferably greater than 250.

The time for producing ammonia in the present invention is appropriately optimized depending on the reaction equipment and other conditions, but for example, when a 0.05 mmol/L solution of molybdenum complex (1a) is used, the TOF can be estimated at 30 minutes.

It goes without saying that the present invention is in no way limited to the above-described embodiments, and can be implemented in various forms as long as they fall within the technical scope of the present invention.

Examples of the present invention are described below. Note that the following examples do not limit the present invention in any way.

[Experimental Example 1] Ammonia production using molybdenum complex (1a) as a catalyst

Ammonia was produced from nitrogen molecules using the above. A 0.05 mmol/L dichloromethane solution of molybdenum complex (1a) was prepared in a Schlenk reaction vessel. Under a nitrogen atmosphere at normal pressure, a dichloromethane solution (500 μL, 25 nmol) of the molybdenum complex (1a) as a catalyst and a tetrahydrofuran solution (5 mL) of diiodobis(tetrahydrofuran)samarium(II) (397.6 mg, 0.725 mmol, 29000 equivalents relative to the molar number of the molybdenum complex) as a reducing agent were added to the reaction vessel, and then a tetrahydrofuran solution (1 mL) of water (13.1 mg, 0.725 mmol, 29000 equivalents relative to the molar number of the molybdenum complex) as a proton source was added, and the mixture was stirred at room temperature of 20 to 25° C. for 30 minutes. Thereafter, an aqueous potassium hydroxide solution (30% by mass, 5 mL) was added to the reaction vessel to stop the reaction. Next, in order to quantify the amount of ammonia generated in this reaction, the reaction vessel was distilled under reduced pressure and the distillate was recovered in an aqueous sulfuric acid solution (0.5 M, 10 mL). The amount of ammonia in the aqueous sulfuric acid solution was determined by the indophenol method. As a result, 8,000 equivalents of ammonia were generated per catalyst (molybdenum complex). The TOF, which is the catalyst turnover frequency, was 267 (1/min).

[Comparative Example 1]
In Comparative Example 1, the catalyst molybdenum complex (1a) was changed to molybdenum complex (6).

The experimental procedure was the same as in Experimental Example 1, except that ammonia was produced from nitrogen molecules. As a result, 4500 equivalents of ammonia were produced per catalyst (molybdenum complex). The TOF, which is the catalyst turnover frequency, was 150 (1/min). The molybdenum complex (6) used in Comparative Example 1 can be synthesized by the method described in the non-patent literature Nature 2019, Vol. 568 (7753), pp. 536-540.

The above results reveal that compared to the TOF of the prior art molybdenum complex (6), which is a catalytic turnover frequency of 150 (1/min), the molybdenum complex (1a) of the present invention has a TOF of 267 (1/min) or more, making it a catalyst capable of high-speed reaction from the initial stage of the reaction.

[Synthesis Example 1]
The synthesis route of the molybdenum complex (1a) used as the catalyst is shown and explained below.

Synthesis of compound (2a)

The synthesis of compound (2a) is shown below. Di-tert-butylphosphine (2.25 g, 14.9 mmol) and paraformaldehyde (450 mg, 15.0 mmol) were added to a reaction vessel, and the mixture was stirred at 60° C. under a nitrogen atmosphere for 16 hours. Thereafter, dichloroethane (150 mL) and 1,2-diamino-4,5-dichlorobenzene (1.07 g, 6.02 mmol) were added to the reaction vessel, and the mixture was stirred at 60° C. under a nitrogen atmosphere for 24 hours. Next, selenium (1.26 g, 16.0 mmol) was added, and the mixture was stirred at room temperature, 20 to 25° C., under a nitrogen atmosphere for 24 hours. The reaction product was concentrated, and the resulting solid was separated by silica gel column chromatography (dichloromethane:hexane=1/1). The collected fraction was concentrated and dried under vacuum to isolate compound (2a) as a white solid in 2.58 g (3.97 mmol, 66% yield).
Melting point = 195.4-196.5 ° C.
¹ H NNR (CDCl ₃ ): δ6.66 (s, 2H), 4.85 (br, 2H), 3.30 (d, J=7.2Hz, 4H), 1.42 (d, J=15.2Hz, 36H).
^13C NNR ( _CDCl3 ): δ137.2 (s), 121.7 (s), 112.4 (s), 37.1 (d, J = 32.6 Hz), 34.6 (d, J = 40.3 Hz), 28.0 (s).
^31P NMR ( _CDCl3 ): δ79.7 (with Se satellites, J=706.1Hz).

Synthesis of compound (3a)

The synthesis of compound (3a) is shown below. Compound (2a) (2.48 g, 3.81 mmol), triethyl orthoformate (10 mL), and ammonium hexafluorophosphate (629 mg, 3.86 mmol) were added to a reaction vessel, and the mixture was stirred in air at 120° C. for 3 hours. The reaction mixture was then concentrated, and washed twice with a mixed solution of dichloromethane (4 mL) and diethyl ether (8 mL), and once with diethyl ether (10 mL). The reaction mixture was dried under vacuum, and compound (3a) was isolated as a white solid in an amount of 2.49 g (3.09 mmol, 81% yield).
¹ H NNR (Acetone-d ₆ ): δ10.69 (s, 1H), 8.69 (s, 2H), 5.57 (d, J=2.8Hz, 4H), 1.50 (d, J=16.4Hz, 36H).
¹³ C NNR (Acetone-d ₆ ): δ144.6 (s), 132.1 (s), 131.7 (s), 117.0 (s), 40.8 (d, J = 26.8 Hz), 39.2 (d, J = 30.7 Hz), 28.0 (s).
^31P NMR (Acetone- _d6 ): δ-143.9 (seq, J=708.3Hz), 83.1 (s with Se satellites, J=732.3Hz).

Synthesis of compound (4a)

The synthesis of compound (4a) is shown below. Compound (3a) (2.58 g, 3.20 mmol), tris(dimethylamino)phosphine (1.5 mL) and dichloromethane (40 mL) were added to a reaction vessel, and the mixture was stirred for 4 hours at room temperature of 20 to 25° C. under a nitrogen atmosphere. The reaction mixture was then concentrated and washed three times with toluene (7 mL). The reaction mixture was dried under vacuum, and compound (4a) was isolated as a white solid in an amount of 1.83 g (2.81 mmol, 88% yield).
¹ H NNR (THF-d ₈ ): δ9.87 (s, 1H), 9.87 (s, 1H), 8.42 (s, 2H), 4.81 (s, 4H), 1.23 (d, J=12.0Hz, 36H).
^13C NNR(THF- _d8 ): δ145.8 (t, J = 12.0 Hz), 132.5 (s), 131.7 (s), 117.1 (d, J = 6.7 Hz), 43.6 (d, J = 28.7 Hz), 32.8 (d, J = 20.1 Hz), 29.5 (d, J = 13.5 Hz).
³¹ P NMR (THF-d ₈ ): δ-146.0 (seq, J=711.9Hz), 24.7 (s).

Synthesis of Molybdenum Complex (1a)

The synthesis of molybdenum complex (1a) is shown below. Compound (4a) (1.30 g, 2.00 mmol), potassium bis(trimethylsilyl)amide (561 mg, 2.81 mmol) and toluene (45 mL) were added to a reaction vessel, and the mixture was stirred for 1 hour at room temperature of 20 to 25° C. under an argon atmosphere. Next, the reaction mixture was filtered through Celite, and trichlorotris(tetrahydrofuran)molybdenum(III) (756 mg, 1.81 mmol) was added, and the mixture was stirred at 80° C. for 26 hours. The reaction mixture was further concentrated to 5 mL, filtered using filter paper, and then dried under vacuum. The obtained solid was washed twice with toluene (5 mL), and then dissolved in dichloromethane (20 mL), and the solution was filtered using Celite. Hexane (30 mL) was gently added to the filtrate, and the mixture was allowed to stand for 5 days to generate crystals. The supernatant in which the crystals were formed was removed, and the crystals were washed three times with hexane (5 mL), and then dried under vacuum to isolate molybdenum complex (1a) in the form of brown crystals (166.3 mg, 0.24 mmol, 13% yield).

The present invention can be used in ammonia production methods.

Claims (3)

The method for producing ammonia is a method for producing ammonia from nitrogen molecules in the presence of a molybdenum complex, a reducing agent, and a proton source, the method comprising the steps of:
The molybdenum complex has the formula (1):

In formula (1), R ¹ and R ² each independently represent an alkyl group having 3 to 6 carbon atoms.
X is an iodine atom, a bromine atom, or a chlorine atom;
^R3 and ^R4 each represent a chlorine atom.
is a molybdenum complex represented by the formula:
the reducing agent is a halide of a lanthanide metal (II);
the proton source is an alcohol or water;
The catalytic turnover frequency of the molybdenum complex is greater than 150;
A method for producing ammonia. The method for producing ammonia described in claim 1, wherein the catalyst turnover frequency is greater than 250. The method for producing ammonia described in claim 1 or claim 2, in which the catalyst turnover frequency can be obtained in a production time of 30 minutes.