Catalyst for ammonia synthesis and method for producing ammonia

Abstract

Provided is a novel catalyst for ammonia synthesis and a method for producing ammonia that uses the catalyst.　Provided are: a catalyst for ammonia synthesis that comprises an oxo complex that includes a PCP-type pincer ligand; and a method for producing ammonia that involves performing an ammonia synthesis reaction using nitrogen, a proton source, a reducing agent, and the catalyst for ammonia synthesis.

Description

Ammonia synthesis catalyst and ammonia production method

[0001] The present invention relates to a catalyst for ammonia synthesis and a method for producing ammonia using the catalyst.

[0002] The Haber-Bosch process, an industrial method for synthesizing ammonia, is an energy-intensive process that requires high-temperature and high-pressure reaction conditions. Therefore, a method for producing ammonia from nitrogen molecules under mild conditions is desired, and in recent years, methods for producing ammonia using specific catalysts have been reported (see, for example, Patent Documents 1 to 3).

[0003] Patent Documents 1 to 3 disclose methods for producing ammonia from nitrogen molecules in the presence of a reducing agent and a proton source, using a trihalide molybdenum complex having a PNP (phosphorus-nitrogen-phosphorus) type pincer ligand or a PCP (phosphorus-carbon-phosphorus) type pincer ligand as a catalyst.

[0004] International Publication No. 2019 / 168093 International Publication No. 2023 / 033185 Japanese Patent Application Laid-Open No. 2023-126984

[0005] As mentioned above, research is being conducted on catalysts for ammonia synthesis under mild conditions, but further research and development of new catalysts for ammonia synthesis is desired.

[0006] In the course of diligently pursuing the above-mentioned research and development, the present inventors discovered that an oxo complex having a PCP-type pincer ligand has activity as a catalyst for ammonia synthesis, and thus completed the present invention.

[0007] Therefore, an object of the present invention is to provide a new catalyst for ammonia synthesis and a method for producing ammonia using the catalyst.

[0008] In order to achieve the above object, the present invention provides the following ammonia synthesis catalyst and ammonia production method, as well as the following oxo complex and synthesis method thereof.

[0009] In this specification, the expression X (numerical value) to Y (numerical value) means X or more and Y or less unless otherwise specified.

[0010] [1] A catalyst for ammonia synthesis comprising an oxo complex having a PCP-type pincer ligand. [2] The oxo complex is a complex represented by the following formula (1) (in formula (1), M is a central metal, R １ and R ２ are each an alkyl group, and X １ is an iodine atom (I), a bromine atom (Br), or a chlorine atom (Cl), and X ２ is the iodine ion (I － ), bromine ion (Br － ), chloride ions (Cl － ), triflate (OTf), nonaflate (ONf), BAr Ｆ ４ (Ar Ｆ : 3,5-bis(trifluoromethyl)phenyl), BF ４ , or PF ６ and R ３ , R ４ , R ５ and R ６ each represents a hydrogen atom, or a linear, cyclic, or branched monovalent hydrocarbon group having 1 to 14 carbon atoms), the catalyst for ammonia synthesis according to [1] above. [3] The oxo complex is a complex represented by the following formula (2): １ and R ２ are each an alkyl group, and X １ is an iodine atom (I), a bromine atom (Br), or a chlorine atom (Cl), and X ２ is the iodine ion (I － ), bromine ion (Br － ), chloride ions (Cl － ), triflate (OTf), nonaflate (ONf), BAr Ｆ ４ (Ar Ｆ : 3,5-bis(trifluoromethyl)phenyl), BF ４ , or PF ６ and R ３ and R ４ each represents a hydrogen atom, or a linear, cyclic, or branched monovalent hydrocarbon group having 1 to 14 carbon atoms), the catalyst for ammonia synthesis according to [1] above. [4] The catalyst for ammonia synthesis according to [2] or [3], wherein M in the formula (1) or the formula (2) is a transition metal. [5] The catalyst for ammonia synthesis according to [4], wherein the transition metal is chromium (Cr), molybdenum (Mo), or rhenium (Re). [6] The catalyst for ammonia synthesis according to [4], wherein X in the formula (1) or the formula (2) is １ is an iodine atom (I), and / or X ２ is iodine ion (I － [7] The catalyst for ammonia synthesis according to any one of the above [2] to [5], wherein R in the formula (1) or the formula (2) is １ and R ２ [8] The catalyst for ammonia synthesis according to any one of [2] to [6] above, wherein each of the alkyl groups having 3 to 6 carbon atoms is a t-butyl group. [9] The catalyst for ammonia synthesis according to [7] above, wherein R in formula (1) or (2) ３ , R ４ , R ５ and R ６

[10] The catalyst for ammonia synthesis according to any one of [2] to [8] above, wherein any one or more of the following is a hydrogen atom:

[11] The catalyst for ammonia synthesis according to [1] above, wherein the oxo complex is a complex represented by the following formula (3):

[11] An oxo complex represented by the following formula (3):

[12] A method for synthesizing the oxo complex according to the above

[11] , comprising reacting a trihalide molybdenum complex represented by the following formula (4) with water in the presence of a catalyst to synthesize the oxo complex:

[13] The method for synthesizing an oxo complex according to

[12] , wherein the catalyst is pyridine.

[14] A method for producing ammonia, comprising carrying out an ammonia synthesis reaction using nitrogen, a proton source, a reducing agent, and the catalyst for ammonia synthesis according to any one of [1] to

[10] .

[15] A method for producing ammonia, wherein the proton source is water and / or the reducing agent is samarium(II) iodide (SmI ２

[16] The method according to

[14] or

[15] , wherein the synthesis reaction is carried out at room temperature under atmospheric pressure in an air atmosphere.

[0011] According to the present invention, it is possible to provide a new catalyst for ammonia synthesis and a method for producing ammonia using the catalyst.

[0012] The following describes in detail the embodiments of the present invention (hereinafter referred to as "the present embodiments"); however, the present invention is not limited to these embodiments, and various modifications are possible without departing from the spirit of the present invention.

[0013] [Catalyst for Ammonia Synthesis] The catalyst for ammonia synthesis according to this embodiment is an ammonia synthesis catalyst made of an oxo complex having a PCP-type pincer ligand.

[0014] (Oxo Complex Having a PCP-Type Pincer Ligand) The catalyst for ammonia synthesis in this embodiment is not particularly limited as long as it is an oxo complex having a PCP-type pincer ligand that exhibits ammonia synthesis activity. For example, an oxo complex represented by the following formula (1) can be used.

[0015] (Oxo Complex Represented by Formula (1)) The catalyst for ammonia synthesis of this embodiment is preferably an oxo complex represented by the following formula (1). In formula (1), M is a central metal; R １ and R ２ are each an alkyl group, and X １ is an iodine atom (I), a bromine atom (Br), or a chlorine atom (Cl), and X ２ is the iodine ion (I － ), bromine ion (Br － ), chloride ions (Cl － ), triflate (OTf), nonaflate (ONf), BAr Ｆ ４ (Ar Ｆ : 3,5-bis(trifluoromethyl)phenyl), BF ４ , or PF ６ and R ３ , R ４ , R ５ and R ６are each a hydrogen atom or a linear, cyclic or branched monovalent hydrocarbon group having 1 to 14 carbon atoms.

[0016] (Oxo Complex Represented by Formula (2)) As the catalyst for ammonia synthesis of this embodiment, an oxo complex represented by the following formula (2) is more preferable. In formula (2), M is a central metal, R １ and R ２ are each an alkyl group, and X １ is an iodine atom (I), a bromine atom (Br), or a chlorine atom (Cl), and X ２ is the iodine ion (I － ), bromine ion (Br － ), chloride ions (Cl － ), triflate (OTf), nonaflate (ONf), BAr Ｆ ４ (Ar Ｆ : 3,5-bis(trifluoromethyl)phenyl), BF ４ , or PF ６ and R ３ and R ４ are each a hydrogen atom or a linear, cyclic or branched monovalent hydrocarbon group having 1 to 14 carbon atoms.

[0017] The central metal M in the above formulas (1) and (2) is not particularly limited as long as it is a metal capable of forming an oxo complex having a PCP-type pincer ligand, but is preferably a transition metal, and more preferably an early transition metal (Groups 3 to 7). Preferred transition metals include, for example, titanium (Ti), vanadium (V), chromium (Cr), manganese (Mn), iron (Fe), cobalt (Co), nickel (Ni), zirconium (Zr), molybdenum (Mo), ruthenium (Ru), rhodium (Rh), hafnium (Hf), tungsten (W), rhenium (Re), osmium (Os), and iridium (Ir). Chromium (Cr), molybdenum (Mo), and rhenium (Re) are more preferred, and molybdenum (Mo) is even more preferred.

[0018] X in the above formula (1) and formula (2) １is an iodine atom (I), a bromine atom (Br), or a chlorine atom (Cl), and is preferably an iodine atom (I). ２ is the iodine ion (I － ), bromine ion (Br － ), chloride ions (Cl － ), triflate (OTf), nonaflate (ONf), BAr Ｆ ４ (Ar Ｆ : 3,5-bis(trifluoromethyl)phenyl), BF ４ , or PF ６ and iodine ions (I － ), bromine ion (Br － ) or chloride ions (Cl － ), and iodine ions (I － ) is more preferable. １ and X ２ are preferably the same halogen, but may be different.

[0019] R in the above formula (1) and formula (2) １ and R ２ are each an alkyl group, preferably an alkyl group having 1 to 12 carbon atoms, more preferably an alkyl group having 1 to 6 carbon atoms, and even more preferably an alkyl group having 3 to 6 carbon atoms. The alkyl group may be a linear alkyl group, or a branched or cyclic alkyl group. R １ and R ２ may be the same alkyl group or different alkyl groups, but are preferably the same alkyl group.

[0020] Examples of the alkyl group include a methyl group, an ethyl group, an n-propyl group, an isopropyl group, an n-butyl group, an isobutyl group, an s-butyl group, a t-butyl group, a cyclobutyl group, an n-pentyl group, an isopentyl group, a neopentyl group, a t-pentyl group, a cyclopentyl group, a 1,1-dimethylpropyl group, an n-hexyl group, an isohexyl group, and a cyclohexyl group, and a t-butyl group is preferred.

[0021] R in the above formula (1) and formula (2) ３ , R４ , R ５ and R ６ are each a hydrogen atom or a linear, cyclic or branched monovalent hydrocarbon group having 1 to 14 carbon atoms. The "linear, cyclic or branched monovalent hydrocarbon group having 1 to 14 carbon atoms" can be selected from the group consisting of, for example, alkyl groups, alkenyl groups, alkynyl groups, aryl groups, alkoxy groups, and phenyl or naphthyl groups which may have at least one substituent selected from the group consisting of a hydroxy group, a halogen atom and a perfluoromethyl group. R ３ , R ４ , R ５ and R ６ It is also preferred that each of R is a hydrogen atom, an alkyl group, an alkoxy group, or an electron-withdrawing group. ５ and / or R ６ is preferably a hydrogen atom, and R ３ and / or R ４ More preferably, is an electron withdrawing group.

[0022] The alkyl group is R １ and R ２ The alkyl group may be the same as those described above, and a t-butyl group is preferred.

[0023] The alkoxy group is preferably an alkoxy group having 1 to 12 carbon atoms, more preferably an alkoxy group having 1 to 6 carbon atoms, and even more preferably an alkoxy group having 3 to 6 carbon atoms. The alkoxy group may be a linear alkoxy group, or a branched or cyclic alkoxy group. R ３ ~R ６ may be the same alkoxy group or different alkoxy groups, but when there are a plurality of alkoxy groups, they are preferably the same alkoxy group.

[0024] Examples of the alkoxy group include a methoxy group, a trifluoromethoxy group, an ethoxy group, an n-propoxy group, an isopropoxy group, an n-butoxy group, an isobutoxy group, an s-butoxy group, a t-butoxy group, an n-pentoxy group, an isopentoxy group, a neopentoxy group, a t-pentoxy group, a 1,1-dimethylpropoxy group, an n-hexyloxy group, an isohexyloxy group, a cyclopropoxy group, a cyclobutoxy group, a cyclopentoxy group, and a cyclohexyloxy group.

[0025] Examples of the electron-withdrawing group include a fluorine atom, a chlorine atom, a bromine atom, an iodine atom, and —CH ２ Cl, -CH=CHNO ２ , a quaternary ammonium group with an anion as a counter ion, a trifluoromethyl group, a perfluoroalkyl group, a trichloromethyl group, a cyano group, a nitro group, a formyl group, a carboxylic acid group, a carbonyl (C １－６ alkyl) group, carbonyl (C １－６ alkoxy) group, carbonyl (Ar ６－１０ aryl) group, carbonylamino group, carbonyl (C １－６ alkyl)amino group, carbonyldi(C １－６ alkyl)amino group, sulfonic acid group, sulfonyl C １－６ Alkyl group, sulfonyl group ６－１０ Aryl group, sulfonylamino group, sulfonyl (C １－６ alkyl)amino group, sulfonyldi(C １－６ alkyl)amino group, Ar ６－１０ An aryl group is preferred. A fluorine atom, a chlorine atom, or —NH ３ cation, -N trimethyl cation, -N triethyl cation, -N dimethylphenyl cation, trifluoromethyl group, -CF ２ CF ３ , -(CF ２ ) ２ CF ３ , -(CF ２ ) ３ CF ３ , -(CF ２ ) ４ CF ３ , -(CF ２ ) ５ CF３ , -(CF ２ ) ９ CF ３ , -(CF ２ ) １１ CF ３ , cyano group, nitro group, formyl group, carboxylic acid group, sulfonic acid group, sulfonylmethyl group and sulfonyltrifluoromethyl group, and more preferably a fluorine atom, a chlorine atom and a trifluoromethyl group.

[0026] (Oxo complex represented by formula (3)) As the catalyst for ammonia synthesis of this embodiment, an oxo complex represented by the following formula (3) is more preferable. The oxo complex represented by the following formula (3) is a compound represented by the formula (1) in which M is Mo and X １ is an iodine atom (I), and X ２ is iodine ion (I － ) and R １ and R ２ is a t-butyl group, and R ３ ~R ６ It is an oxo complex in which all atoms are hydrogen atoms. ｔ "Bu" represents a t-butyl group (tertiary butyl group).

[0027] (Method for synthesizing oxo complex represented by formula (3)) The method for synthesizing the oxo complex represented by formula (3) according to this embodiment includes, for example, a step of reacting a trihalide molybdenum complex represented by the following formula (4) with water in the presence of a catalyst to synthesize the oxo complex represented by formula (3).

[0028] For example, when pyridine is used as a catalyst, an oxo complex represented by formula (3) can be synthesized according to the following reaction formula.

[0029] Water (H ２ The amount of the compound (O) used is preferably 1 to 3 equivalents, more preferably 1 to 2.5 equivalents, and even more preferably 1.5 to 2.5 equivalents, per mole of molybdenum.

[0030] The catalyst may be pyridine, triethylamine, Na ２ CO ３ , Na３ P.O. ４ The amount of the catalyst used can be adjusted as appropriate, but for example, when pyridine is used, the amount is preferably 0.5 to 4 equivalents, more preferably 1 to 3.5 equivalents, and even more preferably 1.2 to 3 equivalents, per mole of molybdenum, which is the central metal of the trihalide molybdenum complex represented by formula (4).

[0031] The reaction solvent is not particularly limited, but examples thereof include cyclic ether compounds, chain ether compounds, nitrile compounds, aromatic hydrocarbon compounds, and saturated hydrocarbon compounds. Cyclic ether compounds are not particularly limited, but examples thereof include tetrahydrofuran (THF), 4-methyltetrahydropyran, tetrahydropyran-4-methanol, and 1,4-dioxane. Chain ether compounds include diethyl ether, diisopropyl ether, 1,2-dimethoxyethane, and cyclopentyl methyl ether. Nitrile compounds include acetonitrile and propionitrile. Aromatic hydrocarbon compounds include toluene and o-xylene. Saturated hydrocarbon compounds include hexane, heptane, and petroleum ether. The solvents may be used alone or in combination. Tetrahydrofuran (THF) is preferred.

[0032] The reaction conditions can be adjusted as appropriate. The temperature is preferably 45 to 75°C, more preferably 50 to 70°C, and even more preferably 55 to 65°C. The pressure does not need to be a pressurized atmosphere, and normal pressure may be sufficient. The reaction time may be selected as appropriate between 1 and 200 hours. When pyridine is used as the catalyst, the reaction time is preferably 1 to 25 hours, more preferably 5 to 20 hours, and even more preferably 10 to 15 hours.

[0033] [Method for Producing Ammonia] The method for producing ammonia according to this embodiment includes a step of carrying out an ammonia synthesis reaction using nitrogen, a proton source, a reducing agent, and the above-described ammonia synthesis catalyst according to this embodiment.

[0034] For example, water is used as the proton source and samarium(II) iodide (SmI) is used as the reducing agent. ２ When using ammonium nitrate, ammonia can be synthesized according to the following reaction formula:

[0035] As nitrogen, nitrogen gas at atmospheric pressure is preferably used. Nitrogen gas is inexpensive, so it may be used in large excess relative to the other raw materials.

[0036] As the proton source, alcohol or water is preferably used, and water is more preferably used. Examples of alcohols include glycol and ROH (where R is a linear, cyclic, or branched alkyl group having 1 to 6 carbon atoms, in which a hydrogen atom may be substituted with a fluorine atom, or a phenyl group which may have an alkyl group). Examples of glycols include ethylene glycol, propylene glycol, and diethylene glycol, with ethylene glycol being preferred. Examples of ROH include linear or branched alkyl alcohols such as methanol, ethanol, propanol, isopropanol, n-butyl alcohol, sec-butyl alcohol, isobutyl alcohol, and tert-butyl alcohol; cyclic alkyl alcohols such as cyclopropanol, cyclopentanol, and cyclohexanol; alcohols containing fluorine atoms such as trifluoroethyl alcohol and tetrafluoroethyl alcohol; and phenol derivatives such as phenol, cresol, and xylenol. The amount of proton source used is preferably 100 to 2,000 equivalents per mole of molybdenum, the central metal of the catalyst used. More preferably, the lower limit can be any value selected from the range of 101 to 170 equivalents, and more preferably, the upper limit can be any value selected from the range of 1999 to 190 equivalents.

[0037] As the reducing agent, a halide (II) of a lanthanoid metal is preferably used. Examples of lanthanoid metals include La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, and Lu, among which Sm is preferred. Examples of halogens include chlorine, bromine, and iodine, among which iodine is preferred. As the halide (II) of a lanthanoid metal, samarium (II) halide is preferred, and samarium (II) iodide is more preferred. The amount of reducing agent used is preferably 100 to 2000 equivalents per mole of molybdenum, the central metal of the catalyst used. A more preferred lower limit can be any value selected from the range of 101 to 170 equivalents, and a more preferred upper limit can be any value selected from the range of 1999 to 190 equivalents.

[0038] The catalyst used is the ammonia synthesis catalyst according to the present embodiment described above. The amount of catalyst used can be adjusted as appropriate, but for example, when water is used as the proton source and samarium(II) iodide is used as the reducing agent, the amount is preferably 0.001 to 0.5 mM, more preferably 0.002 to 0.4 mM, and even more preferably 0.003 to 0.35 mM relative to the reaction solvent. The catalyst may be used alone or in combination of two or more types.

[0039] In the ammonia production method of this embodiment, the synthesis reaction is preferably carried out in a solvent. The reaction solvent is not particularly limited, but examples thereof include cyclic ether compounds, chain ether compounds, nitrile compounds, aromatic hydrocarbon compounds, and saturated hydrocarbon compounds. The cyclic ether compounds are not particularly limited, but examples thereof include tetrahydrofuran (THF), 4-methyltetrahydropyran, tetrahydropyran-4-methanol, and 1,4-dioxane. Examples of chain ether compounds include diethyl ether, diisopropyl ether, 1,2-dimethoxyethane, and cyclopentyl methyl ether. Examples of nitrile compounds include acetonitrile and propionitrile. Examples of aromatic hydrocarbon compounds include toluene and o-xylene. Examples of saturated hydrocarbon compounds include hexane, heptane, and petroleum ether. The solvent may be used alone or in combination. Tetrahydrofuran (THF) is preferably used.

[0040] The reaction conditions can be adjusted as appropriate. The temperature is preferably room temperature, specifically, preferably 5 to 30°C, more preferably 10 to 28°C, and even more preferably 15 to 25°C. The pressure does not need to be a pressurized atmosphere, and normal pressure is sufficient. The reaction time is preferably 0.1 to 10 hours, more preferably 0.2 to 5 hours, and even more preferably 0.5 to 2 hours. The synthesis reaction can be carried out in an air atmosphere.

[0041] [Effects of the Present Embodiment] According to the present embodiment, it is possible to provide a new ammonia synthesis catalyst that enables ammonia synthesis under mild conditions (room temperature, normal pressure, air atmosphere), and a method for producing ammonia using the catalyst.

[0042] Furthermore, according to this embodiment, it is possible to provide an ammonia synthesis catalyst that can maintain ammonia synthesis activity or suppress a decrease in synthesis activity even after storage at room temperature in air, and a method for producing ammonia using the catalyst. The ammonia synthesis catalyst of this embodiment is more stable to air (oxygen) than conventional catalysts (trihalide molybdenum complexes), making it easier to handle. Therefore, strict oxygen (dissolved oxygen) management of solvents and the like used in the synthesis process is not required.

[0043] The present invention will now be described with reference to examples, but the present invention is not limited to these examples.

[0044] <Synthesis Example 1 of Ammonia Synthesis Catalyst (Oxo Complex Represented by Formula (3))> According to the reaction formula below, the trihalide molybdenum complex represented by the formula (4) was reacted with water (2 equivalents / Mo) in THF solvent (60°C) for 13.5 hours using pyridine (2.5 equivalents / Mo) as a catalyst to synthesize the oxo complex represented by the formula (3).

[0045]

[0046] It was confirmed that the synthesized oxo complex exhibited ammonia synthesis activity and could be used as a catalyst for ammonia synthesis.

[0047] Synthesis Example 2 of Ammonia Synthesis Catalyst (Oxo Complex Represented by Formula (3)) An oxo complex represented by the formula (3) was synthesized in the same manner as in Synthesis Example 1, except that the amount of catalyst was reduced from (2.5 equivalents / Mo) to (1.3 equivalents / Mo) and the synthesis reaction time was slightly increased from 13.5 hours to 14 hours.

[0048] It was confirmed that the synthesized oxo complex exhibited ammonia synthesis activity and could be used as a catalyst for ammonia synthesis.

[0049] Ammonia Synthesis Example 1 According to the following reaction formula, ammonia was synthesized by adding nitrogen at atmospheric pressure (1 atm) and SmI as a reducing agent using the oxo complex represented by the formula (3) synthesized in Synthesis Example 1 as a catalyst. ２Ammonia (59 equivalents / Mo, yield 98%) was synthesized by reacting 180 equivalents / Mo of ammonium hydroxide with water (180 equivalents / Mo) as a proton source in THF solvent at room temperature (25°C) for 1 hour.

[0050]

[0051] <Synthesis Example 2 of Ammonia: Evaluation of Stability Against Dry Air> (Synthesis Example A, Comparative Synthesis Example A) Under a nitrogen atmosphere, an oxo complex (0.2 μmol) represented by the formula (3) or a triiodomolibdenum complex (0.2 μmol) having a PCP-type pincer ligand, SmI ２ (360 μmol) and water (360 μmol) were added and reacted for 1 hour to synthesize ammonia, after which the reaction was quenched with dilute sulfuric acid. KOH was added to the solution after the synthesis reaction, and then the solution was distilled off under reduced pressure to obtain ammonia. The obtained ammonia was quantified by the indophenol method, and it was confirmed that ammonia was synthesized in a yield of 99% (Synthesis Example A using an oxo complex represented by formula (3)) or 100% (Comparative Synthesis Example A using a triiodomolybdenum complex having a PCP-type pincer ligand) relative to the added proton source.

[0052] (Synthesis Example B, Comparative Synthesis Example B) In a glove box, the oxo complex (0.2 μmol) represented by the formula (3) or the triiodomolibdenum complex (0.2 μmol) having a PCP-type pincer ligand was placed in a Schlenk flask, and air dried with silica gel was introduced into the flask and the flask was sealed. After leaving the flask for one day, the flask was cooled to room temperature and then cooled to room temperature. After the temperature had risen, the flask was cooled to room temperature and then cooled to room temperature. ２ (360 μmol) and water (360 μmol) were added and reacted for 1 hour to synthesize ammonia, after which the reaction was quenched with dilute sulfuric acid. KOH was added to the solution after the synthesis reaction, and then the solution was distilled off under reduced pressure to obtain ammonia. The obtained ammonia was quantified by the indophenol method, and it was confirmed that ammonia was synthesized in a yield of 71% (Synthesis Example B using an oxo complex represented by formula (3)) or 50% (Comparative Synthesis Example B using a triiodomolybdenum complex having a PCP-type pincer ligand) relative to the added proton source.

[0053] (Synthesis Example C, Comparative Synthesis Example C) In a glove box, the oxo complex (2 μmol) represented by the formula (3) or the triiodomolibdenum complex (2 μmol) having a PCP-type pincer ligand was placed in a Schlenk flask, and air dried with silica gel was introduced into the flask and the flask was sealed. After leaving the flask for one week, the flask was cooled to room temperature and then cooled to room temperature. After the temperature had risen, the flask was cooled to room temperature and then cooled to room temperature. ２ (360 μmol) and water (360 μmol) were added and reacted for 10 minutes to synthesize ammonia, after which the reaction was quenched with dilute sulfuric acid. KOH was added to the solution after the synthesis reaction, and then the solution was distilled off under reduced pressure to obtain ammonia. The obtained ammonia was quantified by the indophenol method, and it was confirmed that ammonia was synthesized in a yield of 97% (Synthesis Example C using an oxo complex represented by formula (3)) or 74% (Comparative Synthesis Example C using a triiodomolybdenum complex having a PCP-type pincer ligand) based on the added proton source.

[0054] As can be seen from the experimental results of Synthesis Example 2 above, the oxo complex of the present embodiment was able to suppress a decrease in ammonia yield (i.e., suppress a decrease in ammonia synthesis catalytic activity) even when left in dry air for one day or one week, as compared to a conventional catalyst (a triiodomolybdenum complex having a PCP-type pincer ligand), and it was confirmed that the oxo complex of the present embodiment has excellent air stability.

Claims

1. Ammonia synthesis catalyst consisting of an oxo complex with a PCP-type pincer ligand.

2. The oxo complex is a complex represented by the following formula (1): (In formula (1), M is a central metal, and R １ and R ２ are each an alkyl group, and X １ is an iodine atom (I), a bromine atom (Br), or a chlorine atom (Cl), and X ２ is the iodine ion (I － ), bromine ion (Br － ), chloride ions (Cl － ), triflate (OTf), nonaflate (ONf), BAr Ｆ ４ (Ar Ｆ : 3,5-bis(trifluoromethyl)phenyl), BF ４ , or PF ６ and R ３ , R ４ , R ５ and R ６ and each represents a hydrogen atom or a linear, cyclic, or branched monovalent hydrocarbon group having 1 to 14 carbon atoms.

3. The oxo complex is a complex represented by the following formula (2): (In formula (2), M is a central metal, and R １ and R ２ are each an alkyl group, and X １ is an iodine atom (I), a bromine atom (Br), or a chlorine atom (Cl), and X ２ is the iodine ion (I － ), bromine ion (Br － ), chloride ions (Cl － ), triflate (OTf), nonaflate (ONf), BAr Ｆ ４ (Ar Ｆ : 3,5-bis(trifluoromethyl)phenyl), BF ４ , or PF ６ and R ３ and R ４ and each represents a hydrogen atom or a linear, cyclic, or branched monovalent hydrocarbon group having 1 to 14 carbon atoms.

4. The catalyst for ammonia synthesis according to claim 2 or 3, wherein M in the formula (1) or (2) is a transition metal.

5. The catalyst for ammonia synthesis according to claim 4, wherein the transition metal is chromium (Cr), molybdenum (Mo) or rhenium (Re).

6. X in the formula (1) or (2) １ is an iodine atom (I), and / or X ２ is iodine ion (I － 6. The catalyst for ammonia synthesis according to claim 2, wherein 7. R in the formula (1) or (2) １ and R ２ and each represent an alkyl group having 3 to 6 carbon atoms.

8. The catalyst for ammonia synthesis according to claim 7, wherein the alkyl group having 3 to 6 carbon atoms is a t-butyl group.

9. R in the formula (1) or (2) ３ , R ４ , R ５ and R ６ The catalyst for ammonia synthesis according to any one of claims 2 to 8, wherein any one or more of 10. The catalyst for ammonia synthesis according to claim 1, wherein the oxo complex is a complex represented by the following formula (3):

11. An oxo complex represented by the following formula (3):

12. A method for synthesizing the oxo complex according to claim 11, comprising reacting a trihalide molybdenum complex represented by the following formula (4) with water in the presence of a catalyst to synthesize the oxo complex:

13. The method for synthesizing an oxo complex according to claim 12, wherein the catalyst is pyridine.

14. A method for producing ammonia, comprising carrying out an ammonia synthesis reaction using nitrogen, a proton source, a reducing agent, and the ammonia synthesis catalyst according to any one of claims 1 to 10.

15. The proton source is water, and / or the reducing agent is samarium(II) iodide (SmI ２ 15. The method according to claim 14, wherein 16. The method according to claim 14 or 15, wherein the synthesis reaction is carried out at room temperature under atmospheric pressure in an air atmosphere.

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